This should help you find an entry point into Project Gutenberg:

http://www.gutenberg.org/wiki/Category:Bookshelf

I don't know how to access it via the main website, but on the mobile website you get information on similar books. As an example:

http://m.gutenberg.org/ebooks/9551.mobile

You will notice it has what other readers downloaded, with the number of downloads of each title; other books by the same author; and other books in the categories that it is filed under.

Now if there were only ways to combine the two approaches.