Quote:Is there any explanation for Book's Alliance status, or is this one of those things that would have eventually been fleshed out if the series was allowed to continue?
There is no explanation. There are hints dropped in various places in both the series and the movie, but nothing to really say what he is/was.
My personal opinion and one I have heard from others as well is that he used to be an operative. The operative in the movie is often refered to as a man of faith. Book says that the operative is somebody that believes hard. I picture book being like this in his younger days. Then something happens and he looses that belief...he loses his faith. So he retires to the abbey to find himself again. Many years later, he has a different outlook on things. There is still part of the operative in him (as seen in the series in one of the eps where river is seeing flashes into people's minds), but it is fought against by his newer understanding of things and whatever belief he gained at the abbey. Eventually he decided that he had been in the abbey long enough; learned all he could there. So he decided to walk about the world for a bit. This is where he meets the crew of firefly.
When viewed like this an interesting circle appears from the series and movie. The series starts with Book on his walk about. He finds the crew and discovers another side of things. Then he moves onto Haven in the movie. Then you have the operative in the movie who at the end loses his faith. You can kind of see him retiring to an abbey after that. Then, many years later, maybe he too will go for a walk about. Thus bringing us back where the series began with Book on his.
None of this has ever really been said officially though. Its all just what some people have come up with for his backstory. It works for me, but it doesn't for everyone. You have to decide for yourself what story fits.
edit: fixed a word