What are you exactly looking for? I think most things can be pulled from
If any of those sites don't have it than go to IRC and they should have it. If IRC doesn't have it than I buy/rent a amazon book and remove DRM from it. AND THE LAST OPTION is getting a physical copy of the book and scanning it and ocr'ing it (you can get a scanner that does both but not cheap) and BAM now you got it. However the last two options are mostly for Indie books, very new books, or textbooks.
I don't know if this was the answer you were looking for.
Eternity (again) Add me on IM
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The ones Eternity listed are my go tos but typically if i can't find the specific book there I just type the books name and then pdf in google. Usually you'll just find the pdf right there in the search results.
Aside from that I would add the Digital Public Library of America, archive.org, and the library of congress to Eternity's list.