Pre-99 Pre-ANH EU is tricky because they play so many elements ambiguously because that was directed by Lucas. Tales of Jedi will certainly give some details about older Jedi and Dark Jedi, and I know the Thrawn Trilogy gives some hints here and there (Spaarti cloning chambers anyone?). Also some interesting stuff would be in the Marvel Comics and early Han Solo novels which I haven't had the chance to get my hands on yet.

Real holy grail would be getting copies of the West End Games sourcebooks used for the Star Wars RPG. Much of that essentially was a canon encyclopedia back in the day.