Shepherd Derrial Book:

When Serenity and the crew stop on Persephone, Mal leaves Kaylee in charge of finding passengers. She notices Book who is looking at the ships rather than the destinations and tells him he's coming with them, since her ship is the nicest. Indeed, he joins Serenity, its crew and passengers, but in the first days his faith is put to trial.

I've been out of the abbey two days, I've beaten a lawman senseless, I've fallen in with criminals. I watched the captain shoot the man I swore to protect. And I'm not even sure if I think he was wrong.

But he stays. And soon reveals that there is more to him than what meets the eye. He can fight, wield weapons and knows a great deal about firearms. He willingly takes part in the mission to save Mal, even though it clearly includes shooting (and even killing people). Book clearly has some obscure past he refuses to talk about. When he is seriously injured by a bullet, Mal contacts the Alliance, who at first are unwilling to help, but upon seeing Book's identification, they take him in immediately. This leads to a lot of speculation. Who exactly is Book.

Quotes:

Book: This is precision work: sharpshooters. From the look of these wounds, I'd say a 54-R sniper rifle, laser sights.
Jayne: You do a lot of shooting at the abbey there, Shepherd?
Book: Rabbits.

If you take sexual advantage of her, you're going to burn in a very special level of hell. A level they reserve for child molesters and people who talk at the theater.

A government is a body of people, usually notably ungoverned.

It's not about making sense. It's about believing in something and letting that belief be real enough to change your life. It's about faith. You don't fix faith, River. It fixes you.

oe: Preacher, don't the Bible have some pretty specific things to say about killing?
Book: Quite specific. It is, however, somewhat fuzzier on the subject of kneecaps.

When I talk about belief, why do you always assume I'm talking about God?

Mal: You seem to know a lot about that world.
Book: I wasn't born a shepherd, Mal.
Mal: You'll have to tell me about that someday.
Book: No, I don't.

(info from Emma-Jane)