Why do children's book series like The Hardy Boys and The Famous Five appeal across generations?

While such books vary in literary reputation, following a saga or enjoying the ongoing adventures of familiar characters led to many children reading for pleasure. The worlds these books created - with their own locations, characters and recurring elements - provided entertainment that was pleasurably predictable yet surprising too.

And many of the books grown-up parents and grandparents enjoyed they now share with their children, or nostalgically collect themselves.

University of Canberra emeritus professor of children's literature Belle Alderman, 83, was a voracious reader as a child, worked as a children's librarian and has read to both her children and grandchildren.

Although there were numerous earlier children's series including L. Frank Baum's Oz, Alderman says children's books and series books in particular really took off after World War II, with more of a focus on what children liked to read as distinct from what adults thought they should read.

"Education took off too," Alderman acknowledges.

Enid Blyton might be regarded as the queen of children's series books. Alderman notes that the prolific British author wrote more than 900 books between 1922 and 1965, ranging from fantasy to adventure to school stories. Her best-known series include The Famous Five - siblings Julian, Dick and Anne, who have adventures with their tomboyish cousin George (who doesn't answer to Georgina) - and her faithful dog Timmy.

Many of Blyton's books remain popular with kids, if not necessarily librarians and other adults, whose criticisms have included that the books are often too simple in their language, repetitive, classist, sexist, and dated.

University of Canberra senior lecturer in education Dr Rachel Cunneen says there have been changes made to Blyton's books including updated covers and illustrations and editing to bowdlerise, update or clarify the texts: for example, Dick, Fanny and Jo in the Faraway Tree books became Rick, Frannie and Joe.

"They're not the best books in the world, but they developed reading habits," Alderman says.

"My view is that if it keeps them reading, they're probably fine."

Both academics think that one of the things that appeals to children about series books is identification. It could the sense of independence and adventure many of the young characters have, especially in our more protective era.

If Blyton is the queen of children's series, then American Edward Stratemeyer might be the king. From 1899 to his death in 1930, through his "Stratemeyer Syndicate" he wrote and oversaw the ghostwriting of hundreds of stories about dozens of characters under various pseudonyms. His daughter Harriet Stratemeyer Adams took over the operation, which was later sold to Simon & Schuster.

Most of the syndicate's creations are now forgotten, like the YMCA Boys and the Outdoor Girls. Perhaps its best-known characters are from two teenage detective series. The Hardy Boys - brothers Frank and Joe (from 1927, books credited to Franklin W. Dixon) and Nancy Drew (from 1930, credited to Carolyn Keene) solved dozens of mysteries and even teamed up on occasion. There have been rewrites that shortened and modernised the books and spin-off series aimed at older and younger readers.

Many such series are largely static - characters age little if at all and the world of the books remains fairly constant, so they don't have to be read in order.

Direct involvement, rather than identification, takes place in the interactive Choose Your Own Adventure books, where the reader - addressed as "you" - makes periodic choices that determine the direction of the stories, which take place anywhere from outer space to the Wild West. Virtue is often, but not always, rewarded and many paths end in death. Life can be random and unfair, a lesson for kids to learn.

And there's the pleasurable frisson of fear in the spooky stories of R. L. Stine's Goosebumps.

In both these series the link between books is thematic - adventure, horror - rather than continuing stories and characters.

Then there's the appeal of teen romances depicted in books such as Sweet Dreams, a spinoff of Sweet Valley High.

"You invest in them," Cunneen says of readers' relationships to series books.

"Children should be able to see themselves in a book and should be able to see others and to understand what other kids experience," Alderman says.

Not that characters need to be human. Michael Bond's Paddington, the marmalade-loving bear from Darkest Peru, starred in more than two dozen books between 1958 and 2018. Paddington's small size as well as his comical capacity to get into trouble are qualities to which children can relate.

Series books both old and new are perennially popular at Lifeline Book Fair, says Elizabeth Hilhorst, a manager of the children's book section. Children look for more recent titles - Harry Potter continues to sell well - and it's usually adults who look for the vintage books.

Second-hand bookstores also sell series books. There's a whole bookcase of them near the front door of Book Lore in Lyneham. Jack Murphy, 26, who's worked at Book Lore for four years, says older men come in to buy books like The Hardy Boys and Biggles. And, Murphy says, Blyton is "eternally popular, even with younger uni students".

He puts it down to nostalgia and he's not unfamiliar with the feeling.

"My father read me Hardy Boys when I was child - they had a patina to them."

Apart from the father-son bonding experience, he enjoyed the above-mentioned "kids on adventure" aspect of the stories.

Alderman says Australian children's literature is "world class" and that the UC Australian children's book collection has 52,000 volumes - among them many series.

One of the major Australian series was the Billabong saga by prolific author Mary Grant Bruce (1878-1958). Starting in 1910 with A Little Bush Maid and ending in 1942 with Billabong Riders, the 15 books centre on Norah Linton from the age of 12. The major setting is the family cattle station, Billabong, in country Victoria.

Norah ages as the series goes on - eventually marrying her brother's friend Wally.

Bruce wrote 23 other books and hundreds of short stories but the Billabong series was her major creation. Her grandson Ian Bruce lives in Canberra and is keeping his grandmother's literary legacy alive. He says her books are not directly autobiographical but there's something of his grandmother - a strong, independent woman - in them. Born in Sale, she spent much of her childhood on her grandparent's and uncles' cattle stations in Victoria.

"Billabong is an amalagam of those places," Ian Bruce says.

As a young adult, Mary Grant Bruce moved to Melbourne and became a journalist and writer. But her connection to the land remained an inspiration.

The 15 Billabong books were last reprinted in 1993 and half of them are now out of copyright. Ian Bruce brought out Australian Christmas Stories, a collection of some of his grandmother's short stories originally published from 1901 to 1945, late last year and runs the website marygrantbruce.com.au.

Other Australian children's series also have a bush connection albeit often a more fanciful one. Ruth Park's Muddle-Headed Wombat books, spun off from an ABC radio series, were published from 1962 to 1971. The title character and his friends, sensible Mouse and vain Tabby, entertained kids for years with their antics. Dorothy Wall's mischievous koala Blinky Bill and May Gibbs' gumnut brothers Snugglepot and Cuddlepie have also endured.

Queensland-born P.L. Travers moved to Britain and stayed there. Between 1934 and 1988 she wrote a series of eight books about the mysterious, magical nanny Mary Poppins who flies in and out of the lives of the Banks family in London.

British theologian and writer C.S. Lewis's allegorical Narnia septet is another well-regarded series.

Sometimes real and fictional worlds appear to blend. Film director Alfred Hitchcock lent his name and likeness to the Robert Arthur-created series The Three Investigators from 1964 until his death in 1980. Arthur wrote 10 of the first 11 books, in which three young friends solved mysteries together. The books have been likened to Scooby-Doo stories: a seemingly supernatural element usually has a mundane, often criminal explanation. The series ran to dozens of books and was particularly popular with German readers.

The reader was encouraged to work alongside Donald J. Sobol's 10-year-old boy detective Leroy "Encyclopedia" Brown (29 books between 1963 and 2012) in solving mysteries before looking up the answers. Most can be worked out through careful reading although some rely on dated information, dubious assumptions and knowing rather obscure facts.

Many of the same things mentioned that appeal in old kids' series continue in the new ones. And adults like series too. Crime fiction, for example, often focuses on particular characters, like Michael Connelly's Harry Bosch. The series habit that is formed early can stay for life and the love of books passes down, with kids experiencing their parents' favourites while discovering their own. And so the cycle continues.

The Lifeline Book Fair is on at Exhibition Park in Canberra on Saturday February 11 and Sunday February 12. Visit lifelinecanberra.org.au.