On the first day of autumn, 1796, a 31-year-old woman named Mary Lamb picked up a carving knife from her family’s London dining table and plunged it into her mother’s chest. Within moments Mrs. Lamb was dead; the police quickly arrived, and Mary was arrested. Perhaps it was testament to those enlightened times (and to the English legal system) that Lamb did not go to the gallows. Instead, she went to a madhouse.

In the semi-rural seclusion of an Islington asylum, Mary Lamb was treated with a combination of restraint, isolation, and drugs, until her mania subsided. (King George III had been treated in similar fashion during his first serious bout of insanity eight years before.) Then she was let go, released to the care of her younger brother Charles.

Little of this would stand out from the historical record had Charles Lamb not become a famous writer. Today’s readers probably know Lamb, if at all, as the coauthor of the 1807 children’s book “Tales from Shakespear,” which he jointly wrote with Mary. But he was a noted essayist, closest friend of Samuel Taylor Coleridge, and an integral part of the circle of Romantic writers who defined Britain’s literary landscape around the turn of the 19th century.

Susan Tyler Hitchcock’s “Mad Mary Lamb” attempts to trace the life of Charles’s sister and collaborator, and to elevate her from a footnote in his career, to the heroine of her own story. According to Ms. Hitchcock, Mary Lamb deserves credit as an author in her own right. In addition to “Tales from Shakespear,” the Lambs published two other children’s books, both of which were chiefly written by Mary. She also acted as a vital support to her unmarried brother – with whom she lived, in Charles’s words, in a state of “double singleness.” As such, Ms. Hitchcock argues, Lamb deserves to be seen as more than merely the appendage of a prominent man, but to “step out into the light” on her own.

This is a noble goal – and similar efforts have recovered many women, particularly literary women, from the condescension of posterity. (Christina Rosetti or Dorothy Wordsworth are good cases in point.) Unfortunately, its subject does not fully live up to the vigorous feminist agenda Hitchcock has set out. Undoubtedly, Lamb’s literary output would have been underestimated by her peers. Her name did not appear on the title page of “Tales from Shakespear,” for instance, until 1838.

Undoubtedly, too, Mary Lamb intersected with a rich and absorbing cast of characters: the poets Wordsworth and Coleridge; the essayist William Hazlitt (who married one of Mary’s best friends); the social critics (and parents of the future Mary Shelley) William and Mary Godwin; the celebrated abolitionist Thomas Clarkson. It is a veritable pantheon of the Romantic literary world. Yet Mary Lamb’s own personal contacts with these figures remain somewhat obscure. Indeed, throughout the book, quotations from Lamb’s own writings are frustratingly rare, leaving her at times little more than a shadow across one of her brother’s pages.

The trouble is that – Ms. Hitchcock’s good intentions notwithstanding – as a writer and companion, Mary Lamb chiefly did live in her brother’s shadow. But there was one thing that did set Mary Lamb entirely apart. That was her insanity. For the rest of her life (which was long – she outlived her brother, to die at 86), and with increasing frequency, Lamb suffered from relapses into lunacy. Onlookers described the poignant pair, walking sadly toward the madhouse, where Charles would shut his sister away until she returned, through the murky workings of treatment and of her own body, to “normal” behavior.

“Mad Mary Lamb” has most to offer by delving into Mary’s strikingly disparate mental and social worlds, between lunatic asylums and genteel family life, mad doctors and literary greats, murder and verse. But it seems implausible to interpret the matricide, as Ms. Hitchcock does, as an act of female liberation that “freed her to explore the rights of women yet to come.”

There is little evidence to suggest that Mary Lamb was either merely the unfair victim of a patriarchal system, or that she consciously manipulated her status to empowering effect. Turning Lamb’s lifetime of lunacy – poorly understood at the time, and hard to reassess in hindsight – into cause for feminist celebration does a disservice to Lamb herself, by leaving her own interior universe frustratingly opaque.

All biographers face an unenviably daunting task: How do you reduce the events of an entire human life into a few hundred pages? A certain kind of historical biography (such as Joseph Ellis’s “American Sphinx” or Stella Tillyard’s marvelous group biography “Aristocrat”) adopts an episodic approach, sketching representative moments in the subject’s life. “Mad Mary Lamb” might have benefited from this perspective. Instead we are given a panorama of an immensely fertile social and cultural milieu, but its mysterious subject still stays hidden in the background.

Ms. Jasanoff last wrote for these pages on the British Raj.