Don't really know how I feel about The Shepard's Tale
tbh. I'll probably need to read it again, but I'm not sure if I'll consider it "canon" as it were when rewatching Firefly
As mainly the whole secret-Browncoat-spy stuff raises more questions than it answers, and the getting-kicked-out-of-the-very-pissied-off-Alliance thing doesn't make sense with how he's treated in the show.
So I'd rather just believe what I imagined before, that Book was a former badass Alliance operative (like the chap in the movie) who then somehow got out of the job and found God. That'll do.