Although Dear Mum is not the first book of letters the actor and Love Your Sister co-founder has curated, he didn't realise he was tapping into such a rich well.

It was a nice surprise for Johnson - "as you get older you get surprised less". Still, there were times he felt he wasn't qualified for the job, and admits that if his sister Connie hadn't died from breast cancer in 2017, it would be her in his place.

"I didn't realise how little I knew about mums," Johnson says.

"Whether it's because I didn't grow up with a mother myself and didn't understand the true significance of mumming or whether I approached the right people who had amazing lives themselves and amazing parents, I don't know.

"The thing is, I've been repping mums for 10 years, ever since my sister got sick in Canberra. I thought I had a handle on this stuff. And I thought as Connie's brother I was the right person because she was a cracking mum.

"I'm just saluting the best mum in Canberra at this point, my sister Connie. If she were in my place, I'm sure the book would be much richer."

Just like the books that came before it - Dear Dad and Dear Santa - it begins with Johnson's own letter, this time to his mum Merrill, who died by suicide when he was three years old.

The letter - which is the longest in the book - describes someone who only knows his mother through the poems that she left behind.

"I grew up with a phantom poet as a mother, in a way. I didn't have a clear picture of her until I found 300 of her poems handwritten in my dad's filing cabinet when I was 21 years old," Johnson says.

"They painted a glorious picture. I know my mum through her poetry, which is kind of romantic and tragic at the same time."

His sister Hilde Hinton's letter, by contrast, is the shortest in the book. It simply reads, "It was all a bit much, wasn't it."

She was 12 when their mum died, and therefore has a different relationship with her mother.

But can't that be said about every parent-child relationship?

Yes and no, Johnson says. While every person and relationship is different, there are some underlying constants.

"There are so many universal truths in all of them that I think they're all relatable," he says.

"I don't have a mum I remember, and I related to every single letter. So that's one of the things that I think makes this collection such a true treasure."

Not all of the mums in the book are biological. Young adult author Nikki Moyes writes to the mother figure who came into her life at 19-years-old.

Meanwhile, author Fiona Scott-Norman writes to her birth mother, who gave her up at the age of 17.

There are letters from people who are close to their mums - such as singer Shannon Noll who affectionately opens his letter with "How are ya darl?" - and there are letters from those who haven't spoken to their mothers in years.

"There's a lot of explaining in the letters - more explaining than I thought there would be. There are many kids out there that feel that their true selves are misunderstood by their mother," Johnson says.

"Denis Walter wishes that his mum had been more understanding when he fell in love with the love of his life when he was 21. She was a divorcee with two children and he's been with her for 44 years, and his stepkids have gone on to have kids of their own.

"It was the right decision then and it's the right decision now. And his mother could never find acceptance of that."

Dear Mum is the furthest thing from a Hallmark greeting card. There isn't a generic "Thank you for being you" message in sight.

The 90 letters are all wholehearted, warm, emotional, and, importantly, honest. And in the case of letters such as Guy Pearce's letter to his mother, who has dementia, it easy to wonder if the letter writers were ever concerned about being too honest.

There is an incredible sense of vulnerability that comes through with Pearce's letter, which describes his struggle in wondering if he should have let his mother go when he had the chance after she had surgery a few years ago.

"I'm sorry, Mum. I'm sorry your doctor was offering you a way out and I didn't see it. I'm sorry you now must endure, more years of oblivion, instead of being reunited with your precious love - my dad, taken from you 44 years ago," Pearce writes.

It's a feeling that anyone with a parent with dementia may find familiar, and often isn't shared with the world.

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"Guy Pearce always feels like he's writing for me or Love Your Sister," Johnson says.

"He forgets that we're publishing these things. Guy's a bit worried about being asked about his mum in every interview he ever has for the rest of his life now.

"But there are also some interesting letters to mums that have passed. I know that, at least in Amanda Keller's case, she, like me, has learned so much more about her mum after death.

"There's an added layer to the contributors that write to a departed mother. There's enough space there to allow for new levels of honesty and new levels of discovery."

The following is Samuel Johnson's letter from Dear Mum.

You'll perhaps consider me a little slipshod opening my letter this way. But it is my letter, and it is my opinion that this correspondence should begin with your words, for yours are so much better.

Softball Match at the Tech School

Frangar

Chook

F***erata

Susie Babe

Danger

Fungoid Fool

Anderson's dick, you can't find it,

Pig's ring

Bullcrap

Stick your arse behind the backstop

and Shut yer mouth

Four balls make a

walk He stole the

whole base Miss;

Miss.

We wanna go

home Strike one

for Fatty Over

there

This kid's trying to kill me

Murder you

F*** you

Take it

easy.

I love that poem. I would love to write one back, but I'm a shit poet.

It feels odd writing to you. I don't know much about you really. I don't know where or when you were born, or when you died. I don't know your middle name or what colour you wore your hair. I thought you might be interested to hear what I do know of you though, for there is much you've missed.

I lived above Dad's second-hand bookshop for years. When I was 15, having abandoned my dreams of tennis, of Wimbledon, I started working at becoming a bohemian of considerable import. I didn't know a great deal about poetry but I'd somehow surmised I wouldn't be able to call myself a true bohemian if I didn't sport at least a rudimentary grasp. I wore my beret, but it was starting to feel false. And so, in search of authenticity, I trundled down our rickety wooden spiral staircase into Dad's bookshop and its poetry section. I found you in the first book I picked out - an anthology of Australian women poets published in the '70s, called Mother, I'm Rooted. I picked it up because it had the word 'rooted' on the cover. I was 15; it figures. I opened the book square on Softball Match at the Tech School and I was in at fungoid fool. You used your words bravely, leaving me giddy and confused. You put pictures in my head and made me laugh and then feel sad straight after. You captured me with your vivid wordplays and punctuated musicality. I hungered for more than your six poems in the anthology. I searched for you in the index of every other book in the section and found nothing. But I had a piece of you. And I could now attest to being a legitimate bohemian, having been so greatly moved by your lyrical dexterity. I donned my beret free of guilt, before taking my customary walk along the beach. To think. And smoke cigarettes. Because that's what 15-year-old bohemians do.

Over the years I'd hear talk of you. Sometimes I'd fossick and sometimes I'd happen upon it. Apparently, you went mad. Too much acid. By all accounts you found life a most dreadful and onerous undertaking, which I think I understand. Despite this, you held famed parties where you would invite the street urchins and the upper classes, just to watch the fireworks. I learnt that you believed in God most fervently, but I can only assume that you shared a tempestuous bond in light of your purported behaviour being so rarely in line with Bible principles. I heard you fed your preschool daughter LSD to open her mind.

When you weren't in the housing commissions you were in the institutions. They don't exist anymore, you might be interested to know. I heard you reached for God when you got lost but he wasn't there. In amongst it all, I'm told you had three kids and married a bloke and it didn't work out. I heard that you loved him more than he loved you and that it broke your heart. But I'm told there were lots of things that broke your heart. I'm told that you finally ended it after countless attempts.

You don't know this, and this is why I'm writing to you, but you came into my home when I was 19. Generally speaking, my memory is patchy, but I remember this day like no other and you feature in it most prominently. I was back for another stint above Dad's bookshop, another effort to reshuffle my ruffled affairs. I had left the bohemian thing behind, realising that only wankers wore berets, and was dedicated to becoming a fully-fledged cynic. I was depressed because the world was like your poems, oddly enough. The one thing holding me together was family and, thankfully, I had care of my nephew Jonathon that day. Young Jonno had been tasked with some creative writing homework. He had to write a story. Dad had encouraged me to write stories from a very early age and he kept every story I ever wrote in his filing cabinet, hoping, as any old book dealer might, that his son would be the next Proust. Regardless, there was one story I had written, when I was about Jonno's age, called The Old Man and the Watermelon and I thought I might read it to Jonno in an effort to somehow inspire him, such was my hubris at that age.

As I rifled through Dad's filing cabinet, I came across a tab that read Merrill's Poems.

There you were again.

My heart faltered. I couldn't deal with it while Jonno was up. I sequestered the file in my room before finding The Old Man and the Watermelon and reading it. Jonno wrote his piece, we had apple pie and custard, he brushed his teeth, I read him another chunk of Huck Finn and tucked him away safe and sound. I sat in my bedroom. I fingered the manila folder nervously, not sure I was ready to see what was inside. It felt wrong to be reading a private file of my father's, but he need not ever know. There were maybe a hundred poems. I turned the pages slowly, edging ever closer to knowing you. I drifted through your twisted mind. Your pages were lyrical, alarming, completely disconnected, manic, paranoid, sinister, even graceful, depending on the day or your mood or your place. In no time flat, there were only two pages left unread. I searched the penultimate page for a title, but this was the first poem without one. It just said, simply and in brackets, (For Samuel Joseph). That's my name. You wrote a poem for me. Before you died. Now you weren't some acid-drenched poet. Now you were my mum. I'm writing to tell you that I found the poem you wrote me. You've probably forgotten it, so here it is:

(For Samuel Joseph)

When 'this old man'

From the nursery

rhyme Rolled home

You were hiding in his sack and

Seizing the first gruff moment his

back Was turned you stuck your

head

Out and grinned

O lord how I sinned

No great sin to

conceive You little

son, receive

A mother's prayer for

you And a fresh pair of

pants Gold ducks on

red overalls

Little fellow fat tum

With your chest all a

rumble Go cough in the

night

Look look see the thumb

Making arcs in the air by

the Window is your own

Thumb and the fingers

Leave the fingers

weaving Greetings to the

pane

And smile just once

again Forever for your

Once and for all mum

If I'd lost you big bonny baby

son Sam Sam listen if you

can Sam There'd never be

another son Like you so

I sent the silly fellow from the

rhyme a Packing, skulking off with

nought But empty sacking for

company

And I kept you

Treasure devil dear

All the seas of joy

Rise to sing for you boy

Surge and swell and

roar All the seas of joy

Sound wonderfully

near Since you've

been here

I wrote to you not just to tell you that I found your poem, and to thank you for it, but in some feeble hope that perhaps, like me, you'll find this letter buried in a filing cabinet somewhere. And so you know who your son is, there are a few things I need to make clear.

I've become a half-decent human. There's a miscreant in me and I've f***ed up plenty, but I've balanced out my base hedonism with sizeable helpings of community work and I endeavour to be the best that I can. They told me I have what you had but I don't believe them, and I've been off my medication for nearly eight years without incident. I've never tried to kill myself. I'm a practising minimalist and your poem is one of my few possessions, protected in fireproof glass. I give most of what I earn to my family and loved ones and am generous with my time. I'm not particularly good at any one thing but I try hard and I'm proud of that. I think you tried hard too. Maybe that's where I got it. Despite mostly feeling that my life has been an unending series of f***-ups, I've found some confidence and a little security, in so far as you can.

I have never blamed you for leaving. Lots of people these days call suicide selfish. They say, what about the kids? What about the family? Not realising it's not at all about them. I don't miss you, for I never had you to miss. That bloke you had three kids with stepped up when you stepped out. He was effeminate and authoritative, so I had a two-in-one type of deal. I'm pretty much okay, except I have intimacy issues and I can't share a bed. I feel lucky that you didn't stick around and f*** up my life like you did my sister's. Hilde found you dead when she was 12. She was late to see you. Ever since, she's carried a swag of neuroses. She's never once been late in the decades since. Thanks for making her so punctual. I've only seen one photo of us together. I've never cared to obtain my own copy. You were cremated, but the small plaque with your name on it is long lost and only your punctual daughter has ever searched for it. To be honest, for that's what letters ought be, I think I did better without you. I think you knew I would. You are mostly forgotten now, which happens of course, but your poetry stays near and that is your gift.

With true thanks, for changing my life for the better, forever, from your, once and for all, big bonny baby son,

Samuel Joseph