For a purported history of the assassination — an “unsanitized and uncompromising ... no spin American story,” as the authors put it, “Killing Lincoln” is sloppy with the facts and slim on documentation, according to a study conducted by Rae Emerson, the deputy superintendent of Ford’s Theatre National Historic Site, which is a unit of the National Park Service.

Other Lincoln experts also have sounded off. In a review published in the November issue of “North & South — The Official Magazine of the Civil War Society,” historian Edward Steers Jr. cites many instances where the book strays from documented history, then asks, “If the authors made mistakes in names, places, and events, what else did they get wrong? How can the reader rely on anything that appears in ‘Killing Lincoln’”?

By taking on Lincoln, O’Reilly and Dugard have set themselves up for avid scrutiny. Few presidents, indeed few subjects, are as voluminously researched and fought over as Lincoln, and have as many amateur and professional specialists eager to display their startling command of minutiae. Steers notes that more than 16,000 books and articles have been written about Lincoln, with more than 125 volumes on the assassination. Oddly, he adds, only eight of the assassination books have been written by professional historians, leaving the field, in his view, to be trampled on by amateurs like Reilly and Dugard.

Eric Foner, a history professor at Columbia who has written about the Civil War, Lincoln and the South for 40 years and whose latest book, “The Fiery Trial: Abraham Lincoln and American Slavery” was showered with awards -- the Pulitzer Prize, Bancroft Prize, and Lincoln Prize -- said he had not read the book in question. But he noted in an e-mail, “I would not be surprised if there were historical errors as [O’Reilly] is better known as a TV polemicist than as a scholar. Of course many people outside the academy have written about Lincoln and the assassination, but all sorts of unproven theories about it abound and one would hope that any writer would make use of all the relevant sources (and avoid historical errors) in writing about the subject.”

“Killing Lincoln” has no footnotes. An afterword on sources lists “books, websites, and other archived information” the authors consulted. But to Steers, the list is inadequate, leaving out important primary documents. “The authors have chosen to write a story based . . . [on] a few dozen secondary books that range from excellent to positively dreadful . . . [with] no vetting . . . treating them as equal,” Steers writes. Among the criteria that earn a book a place in the Ford’s Theatre store are that it is “historically accurate . . . has relevant citations [and uses] primary resources with documentation,” Emerson notes in her report.