1.) Some libraries in my area have occassional 'sidewalk sales', clearing out the 'old' to make room for a continuous stream of newly published works.
2.) Thrift stores often have a huge, albeit hectic, collection of used books. My character is such that I delight in pawing thru thousands of worthless titles in hopes of finding the rare gem. Most people are not so easily amused.
3.) Last, but best: Used book stores. Because their stock is (usually) made up of donated items, they can make a profit despite insanely low prices.
I recently discovered a local store had received 20+ years of Fine Woodworking magazine, no doubt donated by the family of a deceased craftsman. At 25 cents per magazine, they make money and I get a helluva deal on all the back-issues of a magazine I would otherwise never have seen.
This same store sells hardbound books, any size, for $1.50 each. I scored the following for a mere 6 bucks:
'Understanding Wood: A craftsman's guide to wood technology'
by R. Bruce Hoadley
'The Woodworker's Bible'
by Percy W. Blandford
'Tage Frid Teaches Woodworking- Joinery: Tools and Techniques'
by Tage Frid
'Measure Twice, Cut Once: Lessons from a Master Carpenter'
by Norm Abram