Hmm... well after seeing Serenity's Operative, most everyone was pretty sure just what Book's former occupation was, the tactical skills, combat training, administrative level connections, hell it all fit together really, really smoothly. In fact, I'm almost sure they could've kept quite a Book's past, and his role as a former operative would've still be cannon, he just knows way too damn much about how they work, and Atoner is scrawled all over his grizzled face with a big black permanent marker.