During last week's Browncoat Cruise, Ron Glass said Joss had given the OK for him to reveal some tidbits about Book. As follows:
1. There will be a Dark Horse Comic released next year about Book called The Shepherd's Tale
2. A part of Shepherd Book is artificial
3. He found God in a bowl of soup
4. Book is best known for his greatest failure
5. "Derrial" is not his real name. He took that name from a man he killed.
No other details on the comic available so far, but keep checking back.