It bugs the heck out of me that we never learned what Book's story was....
My guess is that he was a former "operative".
I think that Joss Whedon is really doing that on purpose. He does a great job of making the characters real, and in real life, sometimes we never really know someone. I still like to think that Book is a retired Operative. Just a thought.
"It bugs the heck out of me that we never learned what Book's story was...."
Exactly! I sooo wanted to find out what the back story was with Preacher.