Paperbackdigital and baen are my almost exclusive providers. I also use some ereader. Usually twice a year they give away a book a day and I have pickup up a good number of good books that way.
Don't discount public domain books. There are a lot of good books that are public domain.
Another free option is Cory Doctorow at
www.craphound.com. He releases all of his work under a creative commons liscence. I have read several of his books.