First, the good news. "The Cosgrove Report," G. J. A. O'Toole's fictional "solution" to the mysteries surrounding the assassination of Abraham Lincoln, is a humdinger of a book as long as you can keep the old willing suspension of disbelief cranked up. Written in the form of a memoir by Nicholas Cosgrove, a secret detective in the employ of Allan Pinkerton, it captures the tone, the spirit, the minutiae and most importantly the language of the period as few contemporary novels have ever done. In this it is abetted by a number of clever and scholarly footnotes supposedly appended to the manuscript by a present-day detective in the posthumous employ of the last of the Cosgroves.

Either advertently or by a happy accident, O'Toole has stumbled on an obvious but generally overlooked truth: that people's thoughts are reflected in the way they speak and write. The Victorians spoke and wrote very differently than we; therefore, they also thought very differently, and their preoccupations were not the same as ours -- were, in fact, as alien as if they were French or Chinese, sundered from us by whole congeries of experience, sensation and received wisdom that are, by now, all but irrecoverable. It is an interesting, almost eerie concept -- our own ancestors inhabiting a foreign country defined by time rather than geography -- and the fact that O'Toole has somehow caught Victorian nuance and inflection almost perfectly adds immeasurably to the persuasiveness of his undertaking, transporting us to a landscape at once familiar and as exotic as a sinister, murderous Oz.

It is March 1868. A ring of soldiers surrounds the War Department, where Secretary Stanton has been holed up around the clock for 30 days. On the Hill, the Senate sits in solemn judgment over the fate of President Andrew Jackson. These events are related: The impeachment proceedings have been triggered by Johnson's attempt to dismiss the Secretary. And now, for reasons of his own, Stanton has just commissioned young Nicholas Cosgrove to find a man who has been officially dead since 1865: John Wilkes Booth. On the surface, the book then proceeds steadily uphill.

There is the usual midnight disinterment, a damn-your-eyes interview with General Grant, a theatrical (and hilarious) deception of the sort so beloved by the period, wheels turning within wheels, a change of clients, a nocturnal confrontation between Stanton and Johnson in the crypt beneath the Capitol, another nocturnal confrontation in an abandoned tunnel beneath the streets of Brooklyn, a wonderfully improbable aerial pursuit straight out of Jules Verne, and a final surprise that hinges on the discovery of a 114-year-old riding boot. Okay. Why not?

Well, there's the little problem of real people. The fictional use of actual personages is by no means new; historical novelists introduce them all the time, and both Lincoln and Napoleon Bonaparte have enjoyed steady employment as bit players for decades. The current rage is of a rather different sort, resurrectionist rather than historical. In the older mode, the protagonist was a naive observer along the lines of Anthony Adverse or Horatio Hornblower, and the guest stars were deployed for a simple, background purpose: to advance the action or explain it. In these newer books and films, however, the historical personages have become an integral part of the action itself, vigorously participating in the story, saying and doing things at length, and sometimes serving as the central character.

A case can be made that Stanton may have conspired to have Lincoln kidnaped -- he was definitely up to something -- and deserves exposure by whatever means. The fact remains that we don't know whether Stanton ever planned any such thing, and we are extremely unlikely to find out. I don't mean to be spoilsport; O'Toole is nothing if not accomplished. But I remain troubled by an image. Gen. Thomas Eckert, one of the "conspirators" in this book, was undoubtedly a very great rogue and, like all figures who enter the public eye, he was more than a little fictitious even while he lived. But it strikes me that even he deserves better than to be coshed with a bar of magnetic iron in a catacomb under Atlantic Avenue, laid out by a figment in the imagination of a man born long years after he died. Even rogues should be allowed the dignity of their roguery.