Literary biography is not, like literary criticism, a handmaiden of literature. The critic’s goal is always finally to lead the reader back to the work he is discussing; in all but the rarest cases, his own writing is like scaffolding, useful but disposable. For the biographer, on the other hand, the fact that his subject is a writer is in some way irrelevant. The subject serves the biographer simply as a point of entry into a human drama and a social milieu: He or she is the loom on which the biographer weaves a novel that is also a history. If so many biographies are about writers, this is simply for functional reasons: Writers accumulate paper. Even a second or third-rate writer acts as a magnet for correspondence, reminiscence, and anecdote.

From time to time, however, every literary biographer must dream of getting down to biography’s pure essence, by writing about someone utterly unknown to history or literature. (Samuel Johnson boasted that he could write a biography of a broomstick.) In his new book, Michael Holroyd – the author of magisterial lives of Lytton Strachey and George Bernard Shaw, and the dean of literary biographers – both confesses that secret wish and demonstrates why it can never come true.

“Mosaic” (W.W. Norton, 288 pages, $24.95) is a sequel of sorts to Mr. Holroyd’s family memoir, “Basil Street Blues,” in which he used his professional training to unearth the story of his own parents and grandparents. But in “Mosaic,” Mr. Holroyd’s focus shifts from that story itself to the way he constructed it. It is a fascinating essay in meta-biography, exposing the secrets of the biographer’s trade – its tools and its limitations – to public view.

“Basil Street Blues” documented a family history of unusual complexity and intense, though perhaps not that unusual, unhappiness. Mr. Holroyd’s paternal grandfather, Fraser Holroyd, inherited a large fortune from his own father, a major general who was given lucrative shares in a tea company as a reward for saving the lives of the planters during the Indian Mutiny. But Fraser squandered most of this money by investing in an unprofitable glass business – in the 1930s, he was the British agent for Lalique – and by abandoning his wife for a gold-digging mistress. By the time he finally returned to his family, he was on the road to ruin.

No wonder his younger son, Basil, had such a checkered amorous career. His first marriage, to an 18-year-old Swedish beauty named Ulla, lasted just a few years – long enough to produce a son, Michael. Ulla and Basil each went on to have two more marriages and a series of lovers, as their son, adrift and withdrawn, stumbled through misadventures at school, at work, and in the National Service. In recounting this family saga, so full of recriminations and regrets, lost loves and lost fortunes, Mr. Holroyd showed how it equipped him for his vocation. As a biographer, he would be a detached, wary observer, penetrating other people’s mysteries.

To enjoy “Mosaic,” it is not strictly necessary to have read “Basil Street Blues,” but it certainly helps. The method of the new book is to take up characters and plots from Mr. Holroyd’s memoir, and show how they developed in the intervening five years. Sometimes this is simply a matter of the passage of time: Mr. Holroyd’s ancient Aunt Yolande, whose thwarted life was one of the central tragedies of “Basil Street Blues,” died after it was published, and one chapter of “Mosaic” is devoted to his Kafkaesque struggles to settle her estate. In another chapter – a rich and vivid memoir of his first serious lover, the writer Philippa Pullar – Mr. Holroyd offers a stormy sequel to the tale of adolescent sexual deprivation he told in the earlier book.

But the most innovative parts of “Mosaic” are about the craft of biography itself. After publishing “Basil Street Blues,” Mr. Holroyd writes, many readers wrote him to confirm or challenge parts of his story. Surprisingly, the more sensational family anecdotes turned out to be true: It seems one of his cousins was indeed the illegitimate son of Rex Harrison, and his Swedish grandmother really did know Matisse and Picasso. On the other hand, Mr. Holroyd finds, a misunderstanding had led him casually to defame his stepfather’s brother.

Geoffrey Nares, he had written, “had been homosexual, [and] was shot for ‘cowardice’ in the war.” In fact, Geoffrey was not shot for cowardice nor – as Mr. Holroyd’s formulation insinuates – as punishment for his homo sexuality. Rather, he died of a brain tumor while on campaign in North Africa – a fact indignantly brought to Mr. Holroyd’s attention by none other than the father of Camilla Parker Bowles. It is an object lesson in the frailty of biographical truth: Even the least celebrated life, Mr. Holroyd allows us to see, offers a minefield of misinterpretations.

The longest and most interesting section of “Mosaic” is Mr. Holroyd’s attempt to straighten out one of the lingering confusions of “Basil Street Blues.” In that book, he had drawn a symbolic contrast between Aunt Yolande – a spinster whose one great love affair, with a mysterious man named Hazlehurst, was documented only in a few letters – and Agnes May Babb, the much younger woman for whom Yolande’s father had thrown over his wife and children. Yolande, whose desires were sacrificed to her family’s needs, represented England’s obsolescent bourgeois propriety; Agnes May, an irresistible, social climbing adventuress, stood for the rise of a new hedonism.

In “The Search,” the longest chapter in “Mosaic,” Mr. Holroyd sets out to establish the precise connections among Yolande, Agnes May, and Hazlehurst. When he finds out the truth, after a painstaking slog through archives and a series of frustrating interviews, it is hardly sensational. But the sheer effort required to unearth the solution gives the mystery a certain seductive grandeur. There could be no better insider’s view of the biographer’s toil and its unexpected rewards.