In his latest ''bio'' of a fictional 20th-century figure, Seth tackles Canadian talk-show host George Sprott. As with some of his previous subjects, like businessman Simon Matchcard and comics collector Wimbledon Green, it's hard to believe that Sprott never actually existed. Through a Citizen Kane-style mélange of interviews with those who knew him, the emerging portrait has the messiness of reality: an aloof blowhard and a philanderer, but a beloved uncle and man-about-town. Much of Sprottwas serialized in The New York Times Magazine, but this enormous (14'' x 12'') book makes him feel larger than, well, life. A

