Surprisingly, the series is INCREDIBLY short on information about Souls and Soul Society. Short answer: We just don't know. I do agree with the ideas that the main reason most Shinigami are so ignorant about the real world is because they died centuries ago, though, especially in the filler, some characters make a note of saying that "this is my first time in the real world"... and, although I may be misinterpreting it, but it also seems that people can be born in Soul Society; the Kuchiki clan seems to be quite annoyed at adding Rukia... which wouldn't make sense, if all souls came, originally, from Earth; everyone would have to be 'adopted', since there could be no descendants, yes?<br/><br/>Also, there are a lot of references to the fact that most Shinigami don't come from Rukongai; they come from Seireitei. How'd they get there, if not by luck, which shouldn't make any difference? We don't know, but I would presume birth. Not to mention, as the world population grows, wouldn't Soul Society need more and more souls, to send back to the Real World, to inhabit it?  *shrug*<br/><br/>Ahh, yes, I suppose I *should* have mentioned which books are in and not. =P The core three, Player's Handbook 2, and Complete Warrior, Divine, Arcane, and Adventurer. All you can, really, draw is feats, anyway, since spells, gear, and classes are irrelevant. Also; any class with full BaB progression qualifies as a Fighter of their level for qualifying for "Fighter-only" feats.