Books on recreational mathematics can be found at QA95 (Library of Congress) and 793.73/4 (Dewey Decimal System).
Martin Gardner has been mentioned; terrific as his books are, I think those of Ian StewartIan Stewart are even better. Peter Winkler also has a couple of excellent books.
EDIT: See also Assorted Articles on Recreational Mathematics and the History of Mathematics, by Professor David Singmaster, available at http://puzzlemuseum.com/singma/singma-index.htmhttps://www.puzzlemuseum.com/singma/singma-index.htm
Further edit: Jennifer Beineke and Jason Rosenhouse have published three volumes (so far!) of The Mathematics of Various Entertaining Subjects: Research in Recreational Math.