Start with your basics - Barnes and Noble, Borders, and Amazon – shop online or in the store

Bookswaps 

Bookmooch – You give away books to earn credits. Free, but pay to ship the books you give away.
PaperBackSwap – Same as above.
ReadersUnited – Same as above.
FrugalReader – Same as above, but this one has a premium feature that you can pay for.
TitleTrader – Same as above, but also has CDs and DVDs
Bookins – This one requires a $3.99 fee for each book you receive. You don’t pay for shipping though.
WhatsOnMyBookshelf – Same as the first few.
NovelAction – Membership fee of $25 a month, and you pay $4.80 for shipping up to 6 pounds of books.
BookSwapTextbooks
CampusBookSwap
BookHopper
SwapThing
BookCrossing
America’s Book Shelf
Free e-books

  1. Project Gutenberg
  2. Free-ebooks
  3. Download free e books
  4. Many Books
  5. EBookLobby

Other ways to get cheap books

Booksfree – Looks to me like Netflix, same type of payment plan as well.

Flea markets / Swap meets

Thrift stores

Yard Sales