The Phillips Exeter Academy Library in Exeter, New Hampshire. With 160,000 volumes on nine levels and a shelf capacity of 250,000 volumes, it is the largest secondary school library in the world. It is part of the Phillips Exeter Academy, an independent boarding school. A good article about this library can be found here.
Also on these boards
1997 AIA National Twenty-five Year Award Recipient- Phillips Exeter Academy Library in Exeter, New Hampshire; designed by Louis I. Kahn, FAIA