More than once I've had a professor "accidentally" leave a PDF copy of the textbook lying around somewhere, with the URL "accidentally" visible on the projector screen during a break.

Google Books excerpts are enough if you just need to mine citations. Write from Wikipedia or other informal sources—or from just a single book—then find your citations on Google Books by searching for specific info in your paper (you don't want to just cite Wikipedia's citations—some teachers and professors are wise to that). Cut my paper-writing time to less than half, if you factor in the time to go find books in the library. I love libgen, but search on Google Books can take you straight to the page you need. Give it a skim to make sure it's relevant, auto-generate the professor's preferred citation formatting somewhere, done. Works like a charm for light-duty make-work undergrad papers where reading multiple sources is kinda redundant because they're all gonna say basically the same stuff anyway.