Library catalogs are good places to begin looking for books & e-books by keyword, author, title, or subject. Hint: to search the fulltext of millions of books (not all of them available online) use Google Books.
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The books in Geisel Library are arranged by a system called Library of Congress Classification. The letters and numbers ("call numbers") on the label applied to the spine of each book represent this classficiation. The call numbers are used to keep the books in order, arranged by subject and subarranged by author. Books on the relgion are generally shelved with in the range of call numbers beginning with BL through BX.
BL = Religions (general and by region)
BM = Judaism
BP = Islam
BQ = Buddhism
BR = Christianity (especially history of)
BS = The Bible (text and works about)
BT & BV = Theology
BX = Christian Demoniations (BX800+ = Catholicism; BX4795+ = Protestanism)
For more detailed information about how religion books are arranged within the BL-BX classification, go to the offical Library of Congress page.