New York Public Library
Taken as a whole, the entire public library system of the City of New York. But when used in the YW novels, the term normally indicates the former "main library" or "main Research Library" at 5th Avenue and 42nd Street in Manhattan, now known as the Humanities and Social Science Library.
The main branch of NYPL is renowned among wizards as a (clandestine) source of much raw data pertaining to practical wizardry and the Art in general. There are known to be at least two human wizards among the hundreds of staff working at 5th and 42nd. Additionally, there is at least one permanent feline staffer, Ehef; a Senior wizard among the People, he has spent the last six years or so working on the ongoing "porting project" which is intended to make the Book of Night with Moon and the Wizard's Manual available through the NYPL's venerable online cataloging system, CATNYP. (As a side effect of this work, CATNYP's data is already available in the Manual as "assumed content".)
Cats, whether wizardly or not, tend to feel a certain respect for the library because of the two lion-statues flanking its great front stairway...as well as the gap between them. Urruah describes them to Arhu as depicting two feline Powers that Be, the Lion-gods of the Past and Present, Sef and Hhu'au: and -- implicit in the emptiness between them -- the third, invisible Power, Reh-t, the Future. (Ehhif know these two apparent statues as "Patience" and "Fortitude".)