When the Operative (Chiwetel Ejiofor) appeared in Serenity, that’s just the sort of history I imagined for Shepard Book, it would be that trope of Wild West gunslinger turned Preacher kind of thing.Preacher was definitely high up in the Alliance military ranking. Probably lead a double life as well since he wasn't hostile to the browncoats was he?
There have been a few comic collections released for Firefly-Serenity (some written by Whedon) which fill in the blanks (one is purely about Book) which I think are worth a read if you’re a fan - I felt some of the stories tended to suffer a bit from “Babylon 5 Season 4” syndrome, where plot threads need to be ended quickly before their time, but I’ve only read the first 4 collected editions and I’ve just found out there’s been a few more since I bought those. That’s Christmas sorted for me then!