Here are some sites you can try. The books in the Baen free library are not in the public domain but are offered free from Baen Books. Additionally suvudu offers 3 or 4 books for free every month.
http://www.baen.com/library/
http://www.suvudu.com/freelibrary/
http://www.e-book.com.au/freebooks.htm
http://www.gutenberg.org/wiki/Main_Page
oops, just reread the orginal post theese links are for individual books not a zipped archive.