Hi
may I suggest you to use DigitalReading Search that is available at the following address:
http://search.digitalreading.net
and also on
http://www.digitalreading.net
You can search for any topic, all results will be ebooks that are categorized as either free or non-free.
It uses Google to search sites like Project Gurenberg and ManyBooks.net - mentioned above - and others as well.
Regards
Sic.