Decades later, when I wrote a novel that involved Lincoln’s killing at Ford’s Theater, I longed for an equivalent photo — something as sudden, kinetic and revealing. But neither Mathew Brady nor Alexander Gardner had any reason to set up his camera equipment that night; only John Wilkes Booth and his co-conspirators knew that history would be made. Three months later, after a military trial, Gardner had enough notice to train his lens on the scaffold in the yard of Washington’s Arsenal Penitentiary. Swinging from nooses, the bodies of four of Booth’s accomplices, writhing and dying, blurred some of the photographic results.

I read old photographs for unexpected details, such as the faces in the crowd, the people witnessing what a historical novelist can only try to reconstruct. I keep photos around me while I write the way other authors keep music on in the background; they provide not only evidence but a kind of atmospheric stimulation. I don’t think I could have written “Bandbox,” a comic novel about the 1920s, without long exposure to that era’s madcap tabloid photography, and I can’t imagine “Fellow Travelers,” a novel I set during the McCarthy period, without the flash-lit, noirish Weegee photo that went onto the cover of both the hard-bound and paperback editions. For “Watergate,” the details of dress and facial expression in a photo of “the Rose Mary Stretch” — the president’s secretary attempting to re-enact how a gap in one of the Nixon tapes might have been created — told me more about Rose Mary Woods’s agonized embarrassment than any transcript of her testimony could.

Because the historical novelist finally traffics in fiction, not history, it’s his business to enhance and augment pictures, to Photoshop them verbally into a connected exhibition of plausible fakes. A few months ago, researching a novel-in-progress at the Reagan Library in California, I was perusing contact sheets made by a White House photographer on Monday, Oct. 13, 1986, just after the president’s return from his Icelandic summit with Mikhail Gorbachev. Amid all the usual grip-and-grin photos, the moment-by-moment record of presidential bustle, I noticed a quick series of shots in which Nancy Reagan is having an animated and clearly unpleasant exchange with the White House chief of staff, Donald Regan. The pictures, which almost look as if they were captured by a security camera, practically invite the addition of a soundtrack.

Photography hasn’t made me a more visual or descriptive author. (One copy editor who worked on several books of mine pointed out that the only hair color I ever seem to mention is red.) But photographs have given me an energizing sense of verification, an almost childish reason to believe that whatever piece of the past I’m thinking about has actually happened. When I was growing up, we sometimes made the Stations of the Cross during Lent, and it was the sixth of those that truly stopped me in my tracks, the moment when Christ imprinted the image of his face on Veronica’s veil: a miracle. My biblical namesake is the apostle of doubt, and it probably says something that the earliest fictional scene I’ve written takes place in 1845, a decade after the first daguerreotype portraits. Perhaps I’ve gone no further back because I’m afraid to do without photography’s ability to prove the world.

Thomas Mallon’s eight novels include “Henry and Clara,” “Bandbox,” “Fellow Travelers” and “Watergate,” a finalist for the PEN/Faulkner Award. He has also published nonfiction about plagiarism (“Stolen Words”), diaries (“A Book of One’s Own”), letters (“Yours Ever”) and the Kennedy assassination (“Mrs. Paine’s Garage”), as well as two books of essays. His work appears in The New Yorker, The Atlantic Monthly and other publications. A recipient of the Vursell prize of the American Academy of Arts and Letters, for distinguished prose style, he is currently professor of English at George Washington University.