Guide to Buying Books Online
The list below includes some of the Web sites I consult when shopping for books online. Some offer new books at reduced prices, others used books, others "remaindered" books—new books returned from other bookstores. It's worth shopping around!
Amazon® - The trusty standard. Has both new and also used books.
AddALL® - Searches dozens of used book store inventories and compares the prices.
AbeBooks® - Similarly compares used book stores. Has some overlap with Addall, but sometimes better for rarer or out of print books.
half.com® - This is an ebay® site, but it doesn't have the auction component. Sellers are often just people with books to get rid of, not bookstores.
There are many others. This is a start. Enjoy!