Here are some online sources for ebooks:
Project Gutenberg:
(http://www.gutenberg.org/wiki/Main_Page)
there is also Bibliomania:
http://www.bibliomania.com/
and Page by Page books:
http://www.pagebypagebooks.com/
Web literature
http://www.webliterature.net
and The Online Books Page
http://onlinebooks.library.upenn.edu/ (this gives you a list of sites where the title you want is available)
Free Books which contains the full text of many new and popular books that the authors have posted online:
http://newfreebooks.com/