As the original title suggests, What to Audit / A History of Man was written as a guide for Scientologist auditors, pointing out various Space opera "incidents" that are said to have occurred in all of our "past lives".
The book sets out a description of the areas to be audited. It proposes that the human body actually houses two separate entities. The most important is a thetan, the spiritual being said by Hubbard to be the "true self" of a person. According to Hubbard, this is accompanied by a 'genetic entity', or GE, which is "a sort of low-grade soul" located more or less in the centre of the body, and which passes to another body when the current body dies.
Read more about this topic: Scientology: A History Of Man