I hope some day to list my own work here, but in the meanwhile, I'll fill it up with some of my personal favorite authors and other bookish sites.
Buying and selling books...
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Gutenberg Project
The Gutenberg project is a huge attempt to put all public domain literature in plain ASCII text and distribute it freely in an electronic form. This page is an index to Gutenberg documents already available. There are also links here to other Etext sources. - Barnes & Noble
Booksellers. I really appreciate their discount program, it's well worth the fee if you buy a lot of books. - Amazon.com
You can get better prices (mostly) from BN, but Amazon will jook you up with used and out-of-print books as well. If I can't find it on BN, I go to Amazon. - Alibris
Huge source of used and out-of-print stuff. They will also buy select used books.