I've never read the Sin War books (dunno what those are), but I do recall listening to some story bits in the PS version of Diablo. There's supposedly still two guys down in hell that were at the same level of Andariel and Duriel. Forgot what their names were, though.
Belial and Azmodan. I doubt it's about them though. And after looking at the Diablo III website my earlier guess that it would be a prequel was also proven wrong.
Am I the only one who thinks Diablo 1 is better than Diablo 2?
Probably. Don't get me wrong, it does some things well. For example I always felt that Diablo had a darker feel than Diablo II. Still, gameplay wise Diablo I really doesn't give you many options. Hell, your max stats are determined by your class.