I am sure some of you have already tried using Google to find free e-books. At
cebooks blog, I stumbled over the following two Google-queries with some advanced operators, which can do quite some magic:
Find Apache's (default) index page:
+("index of") +("/ebooks"|"/book") +(chm|pdf|lit|zip|rar) +apache
Find a particular e-book file ('TITLE'):
allinurl: +(rar|chm|zip|pdf|lit|tgz) TITLE
Be aware that some of the hits may point to copyright-protected e-books and that neither Google nor I are responsible for that!