ABE books.com
Used, secondhand, rare, and out-of-print books.
Alibris
Search for second hand, out-of-print, and rare books.
Amazon
Books and music at discount prices.
Audible Books
Join, download, and listen to books and magazines. 
 Back in Print
". . . online bookstore making out-of-print books available."
Barnes and Noble
Daedalus Books & Music
"Quality books and music at up to 90% off."
Quality Paperback Books
Reading Group Center
Vintage and Anchor Books--selections for reading groups and  resources to facilitate group discussions