From Arkham Horror Wiki
Rare Book Collection is a special condition card. It first appeared in the Dunwich Horror expansion.
It can only be put into play by an encounter at the Library (or Hall School if the Kingsport Horror and Miskatonic Horror expansions are being employed as well).
Condition: The Library has the following special ability:
Rare Book Collection: Instead of having an encounter here, you may lose 1 Sanity to gain 1 Clue token.