Mr. Tofel is general manager of ProPublica, an investigative journalism organization.

When we think of J. Edgar Hoover in his heyday, it is as a powerful figure of the 1950s and early 1960s. Clint Eastwood's recent biopic, for instance, leaves the impression that Hoover died shortly after Richard Nixon became president in 1969. In fact, Hoover's fatal heart attack did not come until May 1972, when Nixon had been president for more than three years and Hoover was approaching a half-century as director of the Federal Bureau of Investigation.

More to the point, Hoover's death came just a few weeks before burglars, acting on behalf of Nixon's re-election committee, were caught breaking into the headquarters of the Democratic National Committee at the Watergate complex in Washington. Max Holland's "Leak: Why Mark Felt Became Deep Throat" strongly implies that if Hoover had lived just another three or four months, Washington Post reporters Bob Woodward and Carl Bernstein would have been denied their key source—whom they labeled Deep Throat in "All the President's Men" (1974)—and the received history of the Watergate scandal might have been quite different.

Mr. Holland's book is fascinating reading, both when it is convincing and when it is not. His thesis, laid out early on, is that Mark Felt, the bureau's No. 2 man, was motivated to surreptitiously help the Washington Post reporters not by patriotism or a distaste for Nixonian lawlessness—as has been assumed—but by personal ambition....