Radcliffe Camera
Oxford
The Radcliffe Camera was built to house a library. The word "camera" simple means room (contrary to many people's beliefs - including my own, I must admit - it was never a camera obscura!). It can store 600 000 books in rooms beneath the square surrounding the building.
Today it is used as the reading room of the Bodlean Library, most of the books having been moved to other buildings. Unfortunately, it isn't open to the public.