D. B. COOPER: WHAT REALLY HAPPENED by Max Gunther (Contemporary: $14.95; illustrated). Fourteen years ago, D. B. Cooper became the first man to successfully hijack a commercial jetliner. Having threatened to blow up the 727, Cooper asked for and received two parachutes and $200,000. The jet took off with Cooper, the only paying passenger, and somewhere over southwestern Washington State, he jumped off the back steps of the jet into the night and disappeared. The FBI investigated the hijacking for eight years, then quit. The popular belief is that D. B. Cooper died in his jump. Some day some logger will cut down a tree with a skeleton in its top branches. Max Gunther, author of "D. B. Cooper, What Really Happened," doesn't go along with that. Gunther tells a story of Cooper making contact with him through ads placed in the Village Voice shortly after the jump. He suggested they do a book, then dropped out of sight. But 10 years later, a woman contacted Gunther and knew about the newspaper ads. She insisted she had lived with Cooper all those years, that the hijacker had died of natural causes in New York, and she would tell the whole story. Gunther, obviously, believes her story. The FBI agent who worked the case the longest doesn't and thinks it is some kind of hoax. So Gunther's book is, first of all, an explication of the story of how the book came to be written. Then it is a repeat of all the information already written about the actual hijacking. And finally, it is D. B. Cooper's "true" story. Sound exciting? It isn't. Cooper was a dull man who led a dull life both before and after the hijacking. The result is a dull and dumb book that falls somewhere in between nonfiction and speculation, depending on what the reader cares to believe.