That transformation was one that Whitman himself wished to obscure. He said little about the early 1850s, when he hung a shingle as a carpenter in Brooklyn and published almost nothing, working instead on what became the 1855 first edition of “Leaves of Grass.”

Later, he all but disowned his successful 1842 temperance novel “Franklin Evans; or The Inebriate,” and had little interest in seeing his short fiction revived.

“My serious wish,” he wrote in 1882, “were to have all those crude and boyish pieces quietly dropp’d in oblivion.” In 1891, when a critic was planning on republishing some of his early tales, he was blunt: “I should almost be tempted to shoot him if I had an opportunity.”

That doesn’t faze Zachary Turpin, the graduate student at the University of Houston who found the “Jack Engle.” In fact, this is the second time archival lightning has struck Mr. Turpin. Last year, he announced the discovery of “Manly Health and Training,” a previously unknown 47,000-word self-help treatise that Whitman published in The New York Atlas in 1858.

“A friend joked that that’s what would be on my gravestone,” Mr. Turpin said.

The library of lost American literature includes many “known unknowns,” as Mr. Turpin put it (channeling Donald H. Rumsfeld), like Herman Melville’s “The Isle of the Cross” (the eighth and final novel he may, or may not, have finished) and Whitman’s “The Sleeptalker,” a seemingly completed 1850 novel he discusses in his letters, but which does not survive.

Mr. Turpin has made a specialty of looking for the “unknown unknowns,” using vast online databases that compile millions of pages of 19th-century newspapers. One day last May, he entered some names and phrases from fragmentary notes for a possible story concerning an embezzling lawyer named Covert and an orphan named Jack Engle — one of many entries in Whitman’s voluminous notebooks that the online Walt Whitman Archive had deemed to have no clear connection to any known published material.