I presume you mean how he got immediate treatment from the alliance when he was injured and other references as to how he knew about the existence of operatives. Was never explained. Might have been if the show never got cancelled.
Yes this is what I meant. Somebody did elaborate to me about his background as I mentioned but I guess they could have been lying.
Another friend of mine has the theory that Book is the commander that refused to give Mal his support at the beginning of the pilot episode but this seems doubtful given his sway with The Alliance.
After Serenity I got the impression he'd been a former Operative after discussing with Mal how they, well...operate.
I don't think if he'd been a regular member of The Alliance like a soldier or ship captain he'd be so reluctant to discuss it with Mal. Whatever it was he must have done something too bad to talk about.