Dianetics: The Modern Science of Mental Health, which has repeatedly  appeared on bestseller lists, is the recommended beginning book. This is  the definitive work that forms the foundation of Scientology.
Publishers Weekly called Dianetics: The Modern Science of Mental Health “perhaps the best-selling non-Christian book of all time in the West”  and awarded Dianetics its prestigious Century Award for appearing for  more than 100 weeks on its bestseller list.
For those more interested in starting with a broad summary of L. Ron  Hubbard’s findings about Man as a spiritual being, the first recommended  book is Scientology: The Fundamentals of Thought.