Since you're talking pre-renaissance, (and also pre-printing press), all books have to be hand-written (scribed).
For most of the medieval period, the only "caste" that had both time and resources to copy books were the cloistered monks (as opposed to the priesthood in general).
The books were almost entirely theological/religious in nature; the very few secular exceptions were generally histories and primers of the "liberal" arts - reading, maths, and rhetoric - with a rare sprinkling of classic texts (like Plato's Republic).
Regardless, books were *incredibly* expensive. Generally, only rich nobles (and merchants) could afford to own any. Owning a single book marked someone as an educated man; owning more than one a scholar. A large private, secular, library would have a dozen books. The largest private secular library in a *country* would have a few dozen (maybe even a hundred) books. Religious libraries (like that in the Vatican and many abbeys) were usually larger than their secular counterparts, but would be strongly focused on the sacred or theological texts; often they'd have multiple copies of a single work.
None of these were public libraries. None of these were lending libraries. A scholar who needed a particular volume would often need to travel long distances (possibly even through multiple countries) and beg the owner for permission to look at his copy. Large gifts were always welcome, of course.
In short, you can't find any information about medieval libraries because (for all intents and purposes) they didn't exist.