Jeff Hawkins was a Scientologist and member of the Sea Org from 1967 to 2005. He was responsible for 1980s marketing campaigns that brought L. Ron Hubbard's book Dianetics back to the New York Times bestseller lists. Beginning in 2008, he wrote a book-length series of blog posts about his experiences which has led to many further defections from the Church of Scientology. The blog posts have been edited into a hardback book, one of several by long-time high-ranking recent defectors (others include Nancy Many's My Billion-Year Contract, Marc Headley's Blown For Good, and Amy Scobee's Abuse at the Top).
I've read the first few chapters at his blog--it's quite well-written and the comments from others who have shared some of his experiences are fascinating.