Probably the best free site doe eBooks is Project Gutenberg: http://www.gutenberg.org/
If you are using DRM free books (as from that site and other free sites),
you will be much better off using the free Calibre software rather than ADE.
It is much more powerful and much less bug-ridden than ADE.
The only advantage I can think of for ADE is that it can handle DRM eBooks.