One of TV's first superstars, a lead in the series Starsky & Hutch of the '70s, later a movie star and hit recording artist before fleeing Hollywood 15 years ago, David Soul has signed with Transworld in London to do a tell-all memoir, which will be offered to U.S. publishers this week. Soul, who now lives in the U.K. and still makes public appearances, will get together with Carin Davis, London correspondent for People, to write the book, which will tell stories about a host of famous stars and celebrities, according to agent Peter Cox. Cox made the sale, for U.K. and Commonwealth rights (excluding Canada) to Transworld's Patrick Janson-Smith and Doug Young in what he said was one of the biggest memoir sales of the year in London. The book, with a tentative title of It's All True, will be circulating here beginning today.