The Name of the Rose (Umberto Eco) surely? The tantalizing sense of loss as rare texts are censored and erased for ever, leaving as their only trace suggestive references and mentions in other works gets me every time.
I particularly love it because so much of the library was forbidden knowledge guarded by a religious sect... those words almost always mean a good read is ahead :P
Umberto Eco has said he came up with the idea of the library (and some spoiler-related things) before coming up with any of the characters or premise.
