I was all ready to help you until you said "mainstream"
I've bought more books than I can count from Baen, they have very liberal policies: ebooks with no DRM, and even a nicely stocked online library of free books. Jim Baen was smart enough to know that if you give someone the first book of a series for free, you'll be back to buy the rest.
If you like Science Fiction and Fantasy, or are willing to try it, I strongly suggest you go to
http://www.webscription.net/ right now.