Interesting, but I agree it was too cursory. It's nice that the secrets of Book's past aren't what we all expected, but as remarked above, it does raise some continuity questions. I'm assuming this is the backstory Whedon always intended for the character, though.
I didn't much care for the art. I don't like that sketchy style that makes it hard to tell one character from another. In the scene in the cave with the conspirators, I couldn't tell whether I was reading a monologue by the leader or a debate between the leader and some other guy.]
The other thing that bewildered me was the date headings. According to them, Serenity
the movie apparently took place nearly four years after, err, "Serenity" the pilot episode. I thought the movie said it had only been eight months.
I also picked up the Wash one-shot, "Float Out," at the comics store. That was kind of disappointing. The dialogue in the first few pages is painfully stilted with exposition, and the stories about Wash are too cursory. But the last few pages were very moving. Even though I noticed the last page when I was looking through it at the store, so I was technically spoiled, once I came upon the moment in context, it really affected me.