Ah, libraries. One of my favorite things. They are places of power in their own right. All those fine scrolls of velum giving a material shape to thought and knowledge! Oh yes, each library can really help researchs. A library has a number of dedicated fields of knowledge, each more or less complete. The more numerous and trustworthy the books contained, the higher ranked it is.
But the difference between a rich library and an useless heap of book is the librarian. The countless thousands of tomes, grimoires, books, and reports in my Archive Room would be useless waste if they were not cleverly and methodically inventoried, sorted, categorized, ranked, and so on. And with more reports coming everyday from my numerous eyes; and clients wanting to access my informations and trying to deny them to other clients, this would quickly become an unmanageable mess if left to the hands of unskilled librarians.
So, in short, one may posit a library possess skill ranks like a character in a number of Knowledge skills. A library also possess an overall organisation level that depends on its librarian's Profession check (equal to one-tenth the roll result, must be rerolled everytime the whole library is reindexed).
A character making researchs at a library gets a circumstance bonus to his Knowledge check (which he gets to make even if untrained) equal to one per day spent looking for informations, this circumstance bonus being further limited by the library's level and the skill rank in this field of knowledge.