BOOK REVIEW : Literary Sleuth Solves 19th-Century Mystery : MARY DIANA DODS, A Gentleman and a Scholar, <i> by Betty T. Bennett</i> , Morrow $22.95, 303 pps.

“You have brought detection as near an exact science,” Dr. Watson once said of Sherlock Holmes, “as it ever will be brought in this world.”

We might say the same of Betty T. Bennett, a scholar who reveals exactly how she solved a 170-year-old mystery in “Mary Diana Dods, A Gentleman and a Scholar.”

By an investigative and analytical feat of true Sherlockian proportions, Bennett cracks an elaborate conspiracy that had successfully veiled a Pandora’s box of sexual scandal and literary intrigue until Bennett herself revealed it to the world.

Bennett, a professor of literature at American University, specializes in the life and work of Mary Wollstonecraft Shelley, author of “Frankenstein,” among other works, and widow of Percy Bysshe Shelley.

While working on her definitive three-volume collection of Mary Shelley’s letters, Bennett stumbled upon a pair of obscure figures--David Lyndsay and Walter Sholto Douglas--who defied her best efforts to briefly identify them in a footnote.

“The thought of the failed footnote, ‘No biographical information located,’ taunted my scholarly pride,” Bennett writes, “and my curiosity.”

Bennett embarked on an 18-year quest to discover the true identities of Lyndsay, an obscure writer of the early 19th Century, and Douglas, the husband of Shelley’s intimate friend, Isabella Robinson, and the father of her child. As she trudged through libraries and archives, cemeteries and castles, Bennett was tantalized by the intriguing clues that she uncovered--forged passports, secret codicils, illegitimate birth, sham marriage, unmarked graves, all of it enshrouded in the ambiguous sexual identity of the players.

“I felt more a voyeur than ever,” Bennett writes of her own conflicted emotions as a scholar driven by personal passion. “I stepped into a house of mirrors. Inside, moving ahead in the half-light, leading me on, were the three women . . . .”

Three is another clue to the mystery that Bennett finally solves in “Mary Diana Dods.” Mary Shelley is the first women; Isabella Robinson, the putative Mrs. Walter Sholto Douglas, “ever false yet enchanting,” is the second; the third woman is Mary Diana Dods, but I am loathe to give away the plot of Bennett’s literary detective story by saying anything more about her.

Suffice it to say that things are not exactly what they seem in Mary Shelley’s circle, and Bennett’s work as historian and scholar--including the imaginative parsing of old letters and journals, and the meticulous analysis of dates and signatures and spellings and pronoun usage--eventually reveals a truly startling literary and sexual masquerade.

The setting of Bennett’s tale is the sparkling literary circles of 19th-Century London and Paris, and it’s peopled with the celebrities of the era: Lord Byron, Stendhal, Tocqueville, Prosper Merimee. But the sheer number of suspects, accessories and informers in Bennett’s tale--and the complexity of their relations, social and sexual--can be daunting, and it takes some care to follow Bennett as she hastens after her heroes and heroines.

Not incidentally, Bennett manages to give us a sense of the mystery and suspense that motivates the authentic scholar, the visceral pleasure of finding a long-lost letter or a scandalous diary entry in some musty archive--and thereby changing the way history is written. That, in a modest way, is precisely what Bennett has accomplished here.

And she puts the remarkable story of Mary Diana Dods in its historical and literary context; the elaborate deceptions of Dods and her cohorts can be explained, Bennett suggests, by the degradation of women in 19th-Century English society, and the desperate lengths to which a talented woman was forced to go to express herself.

“Being human, they thought; thinking, they knew themselves to be adults. Still, their society insisted otherwise,” Bennett concludes.

“And the most imaginative and courageous might break the code of the realm--and risk the consequences. Among those most daring women were Mary Diana Dods, Isabella Robinson and Mary Shelley.”

Next: Mark Feeney reviews “The Riverkeeper” by Alec Wilkinson (Alfred A. Knopf) .