You might have noticed a link to my BOX.NET epub folder on the right hand column on this site. This is a widget that opens up one folder on the BOX.NET site where I collect EPUB files that I want to keep or share. The epub files on the box.net account are certified free to share. Many are in the public domain, but some are not.
EPUB appears to be emerging as the de facto universal standard for electronic publications designed for e-readers like Kindle, Ipad, Nook, and Ipods. If you want to read EPUB files (such as the ones in the Box account on the right hand side) in your Firefox browser, I strongly suggest the EPUBReader Add-on to the Firefox browser, available here.
This this is outstanding, and even better, free.