As per /u/manwithoutmodem First places to check:

This is from a prior post I have saved from /u/ManWithoutModem

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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)

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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/*

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Academic Torrents (new site)

http://academictorrents.com

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Public Trackers

http://thepiratebay.se/browse/601

http://www.kat.ph/books/

http://bitsnoop.com/browse/other-ebooks/

http://www.filestube.com

There are a lot more public torrent trackers, see here for more.

These last few are sort of specialized eBook private trackers, google them for some info.

http://thegeeks.bz

http://theplace.bz

http://thevault.bz

http://bitseduce.com

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You can also try googling phrases like 'textbook/book title .torrent' or 'textbook/book title .pdf' or 'textbook/book title ebook' or anything along those lines if you are getting desperate, but beware of malware, viruses, etc.

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Private ebook trackers

http://bibliotik.org

http://bitme.org

http://myanonamouse.net (see reddit offers for invite)

http://bitspyder.net

http://www.learnbits.me

http://www.ebookvortex.com

http://elbitz.net

http://docspedia.org

http://abtorrents.com (audiobooks)

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General private trackers have some stuff too, try places like TL or DH, etc, etc. Reddit's unofficial private tracker BaconBits (see /r/baconbits for more info) has a nice collection of eBooks as well. Better than most other private trackers actually.

Don't forget that What.CD and Waffles.FM! They both have a huge selection of eBooks.