Who Was Reverend Book?

One of my favorite TV shows, 2002's martyred Firefly series, had several interesting characters.  It takes place in an imaginary solar system where there are dozens of habitable planets and moons, some of which are high-tech, urban and modern, while others are rural or even backwards.  The show takes place six years after an interplanetary civil war, after which the urban "central alliance" worlds imposed a police state over all the others, but with somewhat limited reach imposed by distance.  The circumstances present many opportunities for interesting contexts for the writers to tell their stories, putting horses and space ships in the same scenes, and allowing all sorts of social circumstances to be considered.  So much more could have been done with the show had it been continued.

One of the interesting characters is Reverend Derial Book.  He had been a monk in an abbey not too far from the Persephone Spaceport--as he puts it "out of the world" for a time, when he catches a ride of the show's namesake spaceship, the Firefly class "Serenity".  He proves very knowledgeable about spaceships and many other things, especially things related to weapons, small force fighting and apparently espionage, and has an identity card which gives him surprising rights in the alliance.  All of this is implausible for the preacher he purports to be.

My theory is that during the war, he was a secret agent for the Alliance, possibly even a highly trained Operative.  After the war, he didn't want to do this sort of work anymore, but was privy to many secrets that needed to be kept.  The deal he worked out with his bosses was that he spend some period--probably five years--completely out of the world, and was prohibited from selling his expert services and knowledge, so he couldn't work as a consultant of some sort.  Moreover, he'd done sufficient bad things that he really didn't want to be in the spy business anymore, but had earned a permanent "get out of jail free" card with the Alliance.  The abbey suited him well--it fit his new-found religiosity and allowed him to be out of the world for a while, but after the five years were up, he could go back out.  He enjoyed space travel and had many skills that would be useful, so he took a ride on what appeared to be a tramp freighter, and was in fact a pirate ship, and after the first adventure, joined the crew.

His background prevents him from telling the rest of the crew what his true history is, especially since the Captain and Zoey had fought against his side in the civil war, but he's realized that his ideas about freedom and civilization are more or less aligned with theirs, and besides, it's exciting and fun for him.  Eventually, he'll probably tell the captain about his activities during the war, but probably not for a long time yet.  As it happens, he was killed by Reavers in the movie Serenity before this occurs.  I regard the events of the movie as non-canonical and perhaps even retcon, although much of the movie does extend the story of the series.  In particular, I think Book and Wash's deaths were intended to terminate the story and make reviving the series impossible rather than being a real part of the story.