For general reference: (could've sworn I posted this already, but can't find the original thread)
A few potentially pertinent links:
~
The 2005 Beginner's Guide to Getting Warez on IRC
~
How to Search for Specific Ebooks With Google
~
Stop Paying for Shit! – The College Edition: A Mini-Manual of Scams, Cons, and Grifts also has an (admittedly brief) section on
"Free Ebooks and Journals"
~
The Reviewer Scam may also come in handy for getting your hands on advance reader copies of books before they're even on sale

.
~
This thread may also have some resources which could prove to be quite useful for like-minded endeavours

.
~There's also a seemingly endless amount of websites devoted to ebooks; b213.net, avaxhome.ru, and gigapedia.org are just a few that come to mind off-hand.
~Most general torrent sites now have ebook sections (for example
here or
here, with
some sites devoted exclusively to ebooks...
Hope this helps point you in the right direction(s)...
Nota Bene 1: Procuring Treeware
In case you also want some ideas on how to score free treeware copies so that you then having something to scan in (making sure not to violate any copyright laws in your jurisdiction, of course), here are a few links to some guides on getting cheap/free treeware in general:
~
How to Get Free (unlimited) Bookstore Credit in 4-8 Weeks
~
Getting Free Magazines
~Again, a couple sections in
Stop Paying for Shit may prove useful, particularly
"Free Books from the Library" and
"Free Textbooks."
~The aforementioned reviewer scam is also as useful for obtaining treeware advance review copies as it is for procuring digital versions
Nota Bene 2: Digitizing Treeware
Should you want to start scanning in your own books, we have a couple guides on hand to cover that too.
Namely,
How to Digitize Books, and the shorter
Free Your Books! A Guide to Scanning Books and Zines (also available for
single-side and
duplex printing), as well as in French for
single-side and
duplex printing).
Be sure to read over the
Security Protocol for Ebook Production first though, so as to protect yourself before engaging in any liberatory activities.
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All of of the above naturally assumes that you either own the copyright to the work in question, or have the copyright holder's permission. It is your patriotic duty to sacrifice everything before the altar of Intellectual Property.
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