The Strand claims to have 18 miles of books and I believe it. This store has deeply discounted books, 50% off cover price on thousands of new titles. They also have books in every category from art books to rare books. Besides their offerings for sale I have known many people who picked up pocket money selling books back to the Strand.
The one thing that seems odd to me about the Strand is that they will sell you books by the foot to decorate empty book shelves but then it is handy if you're dressing a set for a movie or play.
http://www.strandbooks.com/home/
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