Your chances of finding the newest/latest edition of your textbook is much harder than finding an older edition. In my experience, I've always found the edition before the newest one, which my professor undoubtedly will not accept. Might just be my experiences, but I've had trouble finding books of the latest edition.

With that said, here's an excellent collection posted before on reddit before:

General public ebook/textbook resources

http://gen.lib.rus.ec (Use a proxy if you are having issues connecting)

http://textbooknova.com

http://en.bookfi.org/

http://www.gutenberg.org

http://ebookee.org

http://www.manybooks.net

http://www.giuciao.com

http://www.feedurbrain.com

http://oll.libertyfund.org/index.php?option=com_content&task=view&id=380

http://www.alleng.ru/ russian site

http://www.eknigu.com/ russian site

http://ishare.iask.sina.com.cn/

http://2020ok.com/

http://www.freebookspot.es/Default.aspx

http://www.freeetextbooks.com/ need to signup

http://onebigtorrent.org/

http://www.downeu.me/ebook/

http://forums.mvgroup.org (need to register)

http://theaudiobookbay.com/ (audiobooks)

Here's a custom search engine for ebooks: http://www.google.com/cse/home?cx=000661023013169144559:a1-kkiboeco

Sites it indexes:

>http://gen.lib.rus.ec/*

>http://ebookee.org/*

>http://ebooksbay.org/*

>http://free-books.us.to/*

>http://librarypirate.me/*

>http://textbooknova.com/*

>http://www.downeu.com/*

>http://ebookshare.net/*

>http://www.freebookspot.es/*

>http://www.demonoid.me/*

>http://www.kat.ph/*

>http://www.esnips.com/*

>www.4shared.com/*

>http://www.ebooklink.net/*

>http://wowebook.net/*

>http://www.pdfchm.net/*

>http://www.free-ebook-download.net/*

>http://ebookbrowse.com/*

>http://www.ebook3000.com/*

>http://www.ipmart-forum.com/*

>http://www.mediafire.com/*

Academic Torrents (new site)

http://academictorrents.com