Shepherd Derrial Book is a Pastor/Preacher who seems to have vast knowledge of,... shall we say military or Alliance type things.  In addition, he seems to hold special status within the Alliance as in the episode 'Safe' a wounded Book was essentially refused service, until they saw his Ident Card, then he received prompt medical attention.  So, Shepherd Book was one of the characters who was a mystery and I really enjoyed seeing that mystery slowly play itself out during the series, but alas since Firefly was cancelled, most of his story is still a mystery.

I loved the way Ron Glass played Shepherd Book; he was almost always dead calm under virtually any conditions.  The only time I can recall that he kinda sorta broke from the calm was the time River Tam was tearing up his Bible.  If you would have told me, I'm making a show, it's going to be Sci-fi with horses and six-shooters and one of the characters is a preacher;... I'd have said, it's going to be shit.  But, of course, Firefly was great, and Shepherd Book was one of the many reasons.  Although it seemed he didn't get as much screen time as many of the other characters and certainly didn't have as many action scenes, he actually had some of the best lines in the show.