Some books I'll go fetch right away, but for the most part I just keep a giant, numinous web of works associated by recommendations, influences, mood, genre, era and etcetera in my head for reference. There's plenty up there, and always more filtering in, and I could never keep an actual database sorted by so many variables at once
I would need a virtual reality library with books arranged in space to translate the system out of my head. Maybe I could build it in Second Life?