I stumbled across this site while looking for covers on google image search. The site has a large collection of classic science fiction in epub format. Some of the more popular works have been blocked (PKD), but peruse away.
If you do not mind poorly formatted text, you can use Stanza Desktop to convert epub to other formats. If there is a better converter, feel free to share it in the comments. Stanza Desktop’s homepage seems to be dead, but you can search for a download of a legacy copy from several places.