I have a variety of books from a variety of sources:
- Barnes and Noble site
- Baen Books Websubscription
- Mobileread EPUB forum
- some random darknet .lit converts to epub
There are more to try
- Sony's store
- Amazon - shifting format to epub (requires cracking drm)
- Library ebooks
I only recently bought a reader. I waited this long until I felt that the industry is mature enough that I can continue to get my various author's new releases on my reader (a nook). Since it is now standard for the publisher to get digital rights and the infrastructure means releasing that ebook is now just part of the standard process, I was comfortable enough to convert. I don't plan on buying new paper books this year.
I suspect that there will be more not less stores for me to use as the year goes forward. With Calibre I don't feel that I am tied to a given format either.