In addition there are:
-Publishing House Sites: Some publishing houses have set up sites to sell directly to you both print and electronic books. These include the Print-on-Demand publishers who print a copy of a book only after you've paid for it. They almost always offer electronic editions of these books, too.
Who knows what the future holds for "Books"? Our children might not have to read in the future, maybe the information will be downloaded to their brain immediately:P