Lurking within the exotically bedecked, lavishly appointed saloons and antechambers of this palatial work, a novel or a film script eternally seeks to escape. At its opening a small boy watches a famous ballerina dance in a South American theatre and yearns to be the very creature who has so entranced him. By the close, having understandably failed to dance like Pavlova, he has instead gained celebrity as the creator of a brilliantly idiosyncratic choreography whose elegance and wit, entrancing to audiences, have given the Royal Ballet a memorable identity. In the space between, as if this were not already crammed enough with the hero's evident dedication to his art, he contrives to develop two very different private and public faces. As Fred he is the restless pursuer of attractive though not always available men, who may or may not love him in return, while as Freddie he becomes one of those beguilers of tedium, picnic guests and boon companions over cocktails within the orbit of royal intimacy at Clarence House and Kensington Palace.

'Freddie', his grander acquaintances keep assuring Julie Kavanagh (and protesting rather too much in the process), 'wasn't a snob', but a pardonable sense of having made it socially was undoubtedly enhanced by some fairly ordinary family beginnings as the son of a farmer's daughter married to a post office clerk turned business manager. A childhood in Peru lent the required touch of glamour, however, and the positively casebook homosexual formation of dressing up in mummy's frocks, amitiés amoureuses with little girls next door, a joyless stint at a Kent boarding school, was completed with Ashton père's mysterious suicide in Guayaquil when his son was 19.

George Ashton's hopes for Fred can scarcely have involved ideas either of a career as a choreographer at a time when ballet, for all its cachet as the hot new art form, was deemed notoriously unmanly, or of idle hours spent partying among the Bright Young Things with the designer Sophie Fedorovich and the dancer William Chappell, who became the boy's first serious lover. Quietly but sensibly Kavanagh insists that we understand just how much of Ashton's best-known work is informed by this energetic mondanité further fuelled by cosseting from millionairesses like Alice von Hofmannsthal (née Astor), who lured him to bed with the aid of silk shirts 'in Gatsby-like quantities', and frolics amid the let's-pretend-we're-surrealists world of the Sitwells, Lord Berners and Cecil Beaton.

If sex and socialising were itches needing constant scratching, so too was Ashton's creativity as a dance-maker, quite astonishing in its eclecticism, inventiveness and profusion. Among the ballets Kavanagh so dashingly revives for us on the page, from his first success, A Tragedy of Fashion, part of a Nigel Playfair revue at the Lyric, Hammersmith in 1926, to works such as Les Sirènes, an allusive charivari to a pastiche score by Berners, with dancing seagulls and a yodelling Robert Helpmann, much has no doubt dated hopelessly as such things do, yet what would we not give to see The Wise Virgins, that 'daring exercise in stasis', Dante Sonata, hailed at its wartime première as a landmark in English dance, or even the sublime Stravinskian hotchpotch of Persephone?

Genius, as well as his readiness to feed old hatreds, made Ashton enemies as well as admirers. American balletomanes will be unhappy with the image this book projects of their idol Georges Balanchine as a jealous troublemaker, who, despite Fred's evident admiration for him, tried to sabotage rehearsals for Les Illuminations in New York and failed to turn up for the première. Small wonder that when another of his American ballets, Picnic at Tintagel, lost its sets and costumes in a warehouse fire, Ashton sourly observed that Balanchine probably lit it himself.

Kavanagh is likely to be unpopular too with the Does-It-Re-ally-Matter type of biography reader who grows uncomfortable whenever the subject's sexuality is too keenly glanced at. She is marvellously candid in dealing with the sequence of lovers, from Chappell and Waller Gore to the hugely sympathetic Brian Shaw and the altogether more capricious Alexander Grant, if only because the experience of these passions was such an essential inspiration to the choreographer. Of course we don't need to know that the muse of Les Deux Pigeons was a young American Ashton picked up in a bar, or that certain other balletic ideas were prompted by a yearning for the glacially unattainable Michael Somes, but without such details the book, considered purely as biography, would lose its point.

For as such this is a triumph of design, expressiveness and commitment, Ashtonian indeed in the zest and glamour with which Kavanagh invests everything she touches. Any decent chronicle of a life ought to make a reader envious, not simply to have met the subject, but to have lived out in some way the existence described. The best possible tribute to Secret Muses is to say that it makes us long, from start to finish, to have been Frederick Ashton.