The Gutenberg Project has tens of thousands of books available for free download in pdf
http://www.gutenberg.org/wiki/main_page....
If, by chance, there is something you can't find there... then one of my English professors gave me this site, which is basically the same thing and was put together by the University of Pennsylvania:
http://onlinebooks.library.upenn.edu/lis...
Or you can take this route and just search the book you want and enjoy on this site :
http://books.google.com/bkshp?hl=en&tab=...
All copyrighted material is illegal to download. Here are some sites that have non-copywrited material for reading:
http://www.amazon.com/exec/obidos/tg/bro...
http://etext.lib.virginia.edu/ebooks/......
http://www.thefreelibrary.com/
http://www.baen.com/library/
http://www.ebooks3.com/
http://www.free-ebooks.net/
http://e-library.net/free-ebook.htm...
http://truly-free.org/
http://www.hismercy.ca/content/hm_ebooks...
http://www.bookyards.com/
http://www.redbirdstudio.com/e_books/......
http://www.free-online-novels.com/......
http://www.getfreeebooks.com/
http://www.questia.com/index.jsp...
http://www.bibliomania.com/1/frameset.ht...
http://www.starry.com/novel/authors.htm....
http://www.bygosh.com/features.htm......
http://www.bygosh.com/thebestnovels.htm....
http://www.bookspot.com/ask/
http://www.bookspot.com/
But I would try the Gutenberg Project first. I think their collection is more extensive
