Library: before you purchase a book, "test read" it to make sure it is one that you will refer to time and again.
Library: Take advantage of inter-branch loans. In many cities, you can have books sent over to a branch convenient to you for free or a nominal fee.
Become A Book Reviewer: If you have an interest in a certain subject and can do a review on a couple of books for your local newspaper (or even start a blog on a particular subject that is somewhat popular) you can often get review copies of books on that subject for free. This often means getting books even before they are available to the public
Book Exchanges: Check local businesses for book exchanges. You bring in books you no longer want, and take away books they have that you would like to read.
Used Book Stores: For a nice compromise between new and utterly used.
Thrift Stores, Yard Sales, Flea Markets: All offer used books on the cheap. Once you've read them, you might be able to exchange them for store credit at a used book store if they're in decent shape.
Paperbackswap.com - trade your old books for new ones.
Library Sales: Check to see if your local libraries offer an annual sale. Or even a sale shelf. On the annual sales, they often sell the books by the box or bag on the last day.
Online: Half.com, Amazon.com, Ebay.com all have people selling used books.
Craigslist and freecycle: Check out these sites for books being given away and make a request on freecycle!
Read this article for more great ideas "Stop Buying Books" http://www.frugalforlife.blogspot.com/2007/03/stop-buying-books.html