I started with Free Will, a slim volume that explains why free will is an illusion. You can finish this book in a single read, although breaking it up into smaller chunks will make it easier to process. Harris writes: "Thoughts and intentions emerge from background causes of which we are unaware and over which we exert no conscious control. We do not have the freedom we think we have." (Page 5) He spends about 60 very tidy pages providing proofs for this position, and doing some work explaining the implications. Highly recommended.
After taking a break to read some fiction I read Letter to a Christian Nation. This book is a direct response to all the hate mail that Harris got after he published The End of Faith. Like Free Will, this is a very compact, economical book. Harris takes only as many pages as he needs to rebut the absurdity and hypocrisy that surrounds modern evangelical Christianity in the United States. He is not a man to waste words on boring topics, and his cool rationality is a soothing balm if you want to purge the memory of some new and impressively stupid religious news. Highly recommended.