Another option: I've had a great deal of success buying used books online. It helps to buy a few at once, to save on shipping costs.
Besides the library, or a book swap program (see links below), buying used is the next best option and sometimes helps you acquire a title that you can't find otherwise.
Links
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Book swap sites:
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PaperBackSwap
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Swaptree
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BookMooch
Used book sites:
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AbeBooks
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alibris
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half.com books