Bare-Faced Messiah: The True Story of L. Ron Hubbard (1987)

Text version of the book available here.

From the wikipedia article:

Bare-faced Messiah: The True Story of L. Ron Hubbard is a posthumous biography of Scientology founder L. Ron Hubbard by British journalist Russell Miller. First published in 1987, the book takes a critical perspective, challenging the official account of Hubbard's life and work promoted by the Church of Scientology.[1] It quotes extensively from official documents acquired using the Freedom of Information Act and from Hubbard's personal papers that were obtained via a defector.