Which made it all the more astounding when, a few years after his death in 2007, the truth came out. Starting in the early 1960s, Withers had spent nearly two decades as a paid informant of the F.B.I., feeding its agents information about the activists he photographed. He not only informed; he took requests. At one anti-Vietnam War march, he was asked to photograph all of the 30-odd protesters, taking special care to catch all their faces, and he turned 80 8-by-10 prints over to his F.B.I. contact. On occasion, he sold his work to a local paper, then gave copies to the bureau. His daughter Rosalind, the youngest of his nine children and the one who handles his estate, was blindsided when the news came out via a series of FOIA requests and legal fights undertaken by Marc Perrusquia, a reporter from The Commercial Appeal in Memphis. Perrusquia wrote about Withers and the revelation of his intelligence work in his own book, “A Spy in Canaan,” which was published last year. It’s a smart journalist’s book, crisply marching through Withers’s F.B.I. records and the paper’s battle to pry them out of the government’s grip.

“Bluff City,” by Preston Lauterbach, aims instead for something less snappy and more lyrical. Its subtitle is “The Secret Life of Photographer Ernest Withers,” which suggests that it’s a biography, but it isn’t quite that, or at least not a comprehensive one. Big stretches of Withers’s life get a fairly cursory look, and Lauterbach basically calls it a day after King’s assassination in 1968, dispatching the photographer’s subsequent four decades in an introductory chapter and an afterword. Nor is this a book about photography history, examining the photographs the way an art historian might. There are 18 pictures, most, though not all, by Withers, enough to hit the main points but no more. (Which is fine. Decisions about including photos in a book like this tend to be limited by the cost of rights, and anyway there are several nice volumes of Withers’s pictures out there. Or, you know, Google.)

Instead what Lauterbach, a former Memphis resident and the author of two other books set in the South, “Beale Street Dynasty” and “The Chitlin’ Circuit,” is going for is a loose, rangy history of the civil rights movement in Memphis, using Withers and his camera as the (literal) lens. He’s done the work, tracking the complex, intertwined dances of the radicals and the centrists, the local ministers and visiting heavyweights like King. Weirdly, though, his very thoroughness and deep interest in this time and place have the almost certainly unintended effect of diminishing Withers rather than keeping him front and center. There are long stretches where, say, Stokely Carmichael appears, and we get 10 enthusiastic pages about his politics and S.N.C.C. and the dynamics between Carmichael and King, and then Withers pokes his head in to snap a few pictures and go meet his F.B.I. contact. Some of those scenes are nicely wrought, but the secrets in this life are often other people’s rather than Withers’s own.