Shortly after the Civil War began, Allan Pinkerton offered President Abraham Lincoln his services and those of his 18 employees to spy on Confederate traitors, hand-deliver sensitive White House communications and identify disloyal employees in the federal bureaucracy. Pinkerton became spymaster for Gen. George McClellan. In time, other secret agents served the Union cause.

Lafayette Baker reported to Secretary of War Edwin Stanton. George Sharpe served Gens. Joseph Hooker, George Meade and Ulysses S. Grant. Elizabeth Van Lew, referred to in government documents as "a lady in Richmond," helped slaves and imprisoned Union soldiers escape, deployed her operatives throughout the city to gather information and used her Southern mansion as the underground's base station.

In "Lincoln's Spies," Douglas Waller ("Disciples: The World War II Missions of the CIA Directors Who Fought for Wild Bill Donovan") provides a lively account of the activities of this quartet and their stables of secret operatives. Waller acknowledges that Pinkerton was inept, that Baker was corrupt, that "practically every step in their collection and analysis of military intelligence was filled with error" and that their memoirs are riddled "with factual errors, embellishments, and fabrications." He maintains, however, that Van Lew and Sharpe (who developed "all-source intelligence" — merging espionage, Army and balloon reconnaissance, signal intercepts and interrogations of prisoners, deserters and refugees) "made a difference."

Impressively researched, "Lincoln's Spies" illuminates a little-known aspect of the history of the Civil War. That said, Waller has chosen a challenging subject. Operating in the shadows, spymasters often communicate information verbally; they depend on sources whose credibility is difficult or impossible to verify, and whose observations, with reference to troop movements, have a short shelf life. And Waller's spymasters bragged. A lot.

Waller struggles at times to specify when and how spymasters substantially improved the tactical or strategic calculations of Union generals.

It is not entirely clear, for example, that Sharpe "contributed a great deal to the outcome" of the battle of Gettysburg. And, Waller acknowledges, even excellent spymasters make lots of mistakes. For this reason, no doubt, Hooker lost confidence in Sharpe. Meade also had "a falling out" with him shortly after the victory at Gettysburg. Complaining that he was having "great difficulty in getting reliable information," Meade wrote to his wife that he had to grope his way in the dark — and suspended Sharpe's all-source intelligence system. And Sharpe admitted that Jubal Early's raid on the suburbs of Washington, D.C., had been a humiliating intelligence failure.

On the other hand, Grant greatly valued the information Sharpe supplied. When Grant became president in 1869, he rewarded his spymaster with plum jobs in the federal government.

And so, on balance, Waller may be right to conclude that Sharpe was, indeed, a secret service pioneer.

It seems reasonable to conclude as well that in "Lincoln's Spies" Waller does a fine job of supplying vivid portraits and judicious assessments of the larger-than-life characters who ran intelligence-gathering, espionage and counterespionage operations that were much better than anything the Confederates could muster.

Glenn C. Altschuler is the Thomas and Dorothy Litwin professor of American studies at Cornell University.

Lincoln's Spies
By: Douglas Waller.
Publisher: Simon & Schuster, 595 pages, $35.