A long-time Washington guessing game ended in May. The family of retired FBI official W. Mark Felt announced that he was Deep Throat, the celebrated anonymous source for a newspaper series covering the Watergate scandal in the early 1970s. With Mr. Felt's help, Washington Post reporters Bob Woodward and Carl Bernstein investigated the cover-up of illegal campaign activities that led President Richard Nixon to resign from office in 1974. Now Mr. Woodward offers his perspective on that relationship in a new book called The Secret Man: The Story of Watergate's Deep Throat.

Bob Woodward and Carl Bernstein wrote about Deep Throat in their 1974 book about Watergate, called All the President's Men. He also appeared in the movie version of the book, portrayed by Hal Holbrook as an unnamed figure wrapped in shadows. But for more than 30 years, Bob Woodward stuck to his vow to reveal Deep Throat's identity only after his death.

Once the secret was out, he rushed into publication the book he had been holding back for years. He told NBC News that he could finally tell "the very complicated three-decade story that involved lots of uncertainty and anguish." And he suggested that the story has contemporary relevance as well. "In a time of security and surveillance," said Mr. Woodward, "which is the period we live in after 9-11, for reporters to be able to have a confidential source like that, somebody who really knows what's going on in the center of things, that's vital."

In The Secret Man, Bob Woodward describes how he first met Mark Felt in a White House waiting room in 1969. Then a young Navy lieutenant, Mr. Woodward was there to deliver a package. Mr. Felt was an assistant FBI director, and soon became a career mentor to the young Woodward. On June 17, 1972, 5 men were arrested for burglarizing Democratic National Committee offices in Washington's Watergate office building. By now a Washington Post reporter, Bob Woodward and his partner Carl Bernstein painstakingly pieced together a story involving attempted eavesdropping and sabotage of Democratic Presidential candidates.

Mr. Woodward writes that his secret conversations with Mark Felt confirmed information the 2 reporters already suspected, or directed them to new leads. The FBI official's motives for providing that help were complex, Mr. Woodward told CBS News. "He clearly saw lots of lawbreaking in the Nixon White House," Mr. Woodward said. "He didn't like the fact that the Nixon White House was trying to push the FBI around and politicize it. He was also upset that he had not been made director of the FBI after Hoover died. But, by and large, Mark Felt and the other sources Carl Bernstein and I had had it right. In fact, they understated the level of criminality in the Nixon White House."

Mark Felt had worked in espionage surveillance earlier in his FBI career, and his dealings with Bob Woodward acquired all the trappings of a high-stakes spy drama. The young reporter would move a flower pot containing a red flag to the back of his apartment balcony when he wanted to arrange a meeting with Mr. Felt. Those meetings took place in the dead of night in a parking garage. As the number two man at the FBI, Mark Felt was taking risks Bob Woodward calls "immense." "That's why he insisted that we have these really extraordinary and unheard-of clandestine meetings in an underground parking garage," Mr. Woodward says. "What he wanted to do is guarantee that he would not be fingered, that I would protect him and no one else would know."

A Washington Post editor nicknamed Mark Felt Deep Throat, inspired by the title of a pornographic movie of the time. The name was a play on the journalistic term "deep background," meaning information used in a news report without identification of the source. Mark Felt was determined to make sure he remained unidentified, even after the Watergate story broke. "When Carl Bernstein and I wrote All the President's Men, we wanted to disclose his identity," Mr. Woodward recalls. "Felt absolutely insisted, no. It turned out later he was investigated and prosecuted for authorizing break-ins by the FBI, so there are lots of ironies in this story, and lots of anguish also."

President Reagan pardoned Mr. Felt for those break-ins, which involved the radical Weathermen's group. He and Bob Woodward lost touch until they met again in California in 2000. By that time, writes Mr. Woodward, Mark Felt was suffering from memory loss and claimed to remember almost nothing about Watergate. When his family announced that he was Deep Throat, Bob Woodward was caught by surprise. "I didn't think he would, but his family and lawyer decided to do it," Mr. Woodward says. "He seems happy now. He's lost his memory. But there's a smile there, and I haven't seen a smile on his face for a long, long time."

Bob Woodward's book appears at a time when the question of how journalists should protect secret sources is again in the news. New York Times reporter Judith Miller went to jail rather than reveal what she knew about who might have leaked the identity of CIA undercover agent Valerie Plame. Mr. Woodward calls Judith Miller's actions "courageous." "I think the judge and the special prosecutor in this case made a mistake," Mr. Woodward says. "If they'd really looked at it, there was a way to compromise. And to sever the public from that kind of story about what's really going on is a big, big mistake. And we'd better wake up to the chilling impact."

For Bob Woodward, the secret source that helped launch his career has left a lasting legacy. He has published numerous books since that time on everything from Hollywood's drug culture to the Supreme Court to the inner workings of the Bush White House. He writes in The Secret Man that those he has interviewed over the years repeatedly agreed to talk anonymously because they knew he would protect them, just as he had protected Deep Throat. In The Secret Man, he finally reveals the details of a relationship that helped bring down a Presidency and marked a milestone in American journalism.