addall.com

The book search engine that allows you to comparison shop more than 40 online bookstores including Amazon, Barnes and Noble, and Borders.

LINK: http://www.addall.com/

books.google.com

Google Book Search the power of Google to find books.  It is a tool from Google that searches the full text of books. 

LINK: https://books.google.com/

freebooksearch.net

Free Book Search is a comprehensive book searching portal with more than 30 search engines in its archive. The site searches hundreds of digital libraries and also scours the net for hidden books.

LINK: http://www.freebooksearch.net/