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Why The Animals? Because in their correspondence from 1956 to 1970, Christopher Isherwood and his young lover Don Bachardy assumed the playful roles of, respectively, Dobbin the Horse and Kitty the White Cat.

Their relationship, which began in 1952, when Isherwood was 48 and Bachardy 18, was occasionally tempestuous but at other times deeply affectionate. The fact each simultaneously had numerous other lovers made little difference to their mutual commitment.

There are fascinating parallels and significant differences here with the relationship between the 45-year-old Oscar Wilde and the odious 20-something Lord Alfred Douglas (''Bosie''). However, in the Isherwood-Bachardy relationship, there is no hint of tragedy. Instead, there's a mutual commitment that lasted until Isherwood's death.

One reason their partnership endured was, paradoxically, because of their frequent separations. Together, there were often tensions. ''A cruel truth about the Animal personae,'' Isherwood writes, ''is that they could be deployed falsely, thereby maintaining a sentimental fiction of harmony when there was none.''

While Isherwood pursued his calling as a writer during the period covered by this correspondence, producing some of his finest novels, such as A Single Man, A Meeting by the River and Down There On a Visit, Bachardy emerged as a celebrity portraitist, his sitters including movie stars and leading intellectuals and public figures.

The letters show that he, too, is a talented writer, if not in Isherwood's class. Indeed, they collaborated on more than one project, including a stage version of A Meeting by the River and a television mini-series, Frankenstein: The True Story.

Their communications are intensely mannered and private. Reading them, we frequently feel like intruders. They are probably best enjoyed piecemeal to minimise their occasionally cloying, precious tone.

An extract from a 1961 letter reads: ''What a remarkable, irreplaceable and adorable darling Kitty's old Horse is.''

They do, however, provide intriguing glimpses, as do Isherwood's diaries, into the celebrity-studded world in which he and Bachardy moved: they counted Igor Stravinsky, Cecil Beaton, E. M. Forster, David Hockney, Stephen Spender, Truman Capote, Gore Vidal and George Cukor among their closest friends and, as revealed in the 2007 documentary Chris & Don: A Love Story, they often made the A-list at fashionable Hollywood parties. Helpfully, the editor of the letters includes a list of frequently used first names, together with their surnames to aid the reader.

Despite these friendships, the correspondents often privately expressed reservations about people they both knew. Bachardy, for example, is scathing about Beaton: ''In a funny kind of way, Cecil's not a gentleman. He is much too self-centred to be really concerned with people beyond the outward forms of politeness, and these keep slipping, particularly under close observation, and I see enormous selfishness and more than a little vulgarity in him. His sensitivity, if exposed to anything unrefined or unpretty, is exaggerated to the point of hysteria. To spend a night or two, for instance, in an ordinary hotel room, becomes exquisite torture for him.''

Isherwood was probably best known as the author of The Berlin Stories, which eventually became the basis for the hit Broadway musical Cabaret. At first disliking it, he revised his judgment when the income it generated provided him with lifelong financial security.

The last brief letter here is from Isherwood to Bachardy, dated May 18, 1970, congratulating him on his birthday. They nevertheless remained together until Isherwood's death at 81 on January 4, 1986.

As Isherwood was dying of prostate cancer, Bachardy executed daily drawings of him, reproduced in a unique volume, Christopher Isherwood: Last Drawings (1990). At times harrowing and repellent, these drawings nevertheless represent Bachardy's finest work and definitively establish his claim to be considered a significant artist.

For The Animals, he extended to its editor, Katherine Bucknell, his fullest co-operation. Her presentation is immaculate, enhanced by footnotes and an insightful introduction. Having edited four volumes of Isherwood's diaries, she is now preparing a major Isherwood biography. It will almost certainly be definitive.