I remember the times when my monitor stand was "made" out of yet-to-be-recycled paper books - a few thick java references. Later, when I bought a new monitor I donated them to a library. I hope they got recycled, or at least, garbage-collected. Although I can't imagine anyone finding old java books in the garbage and find them useful for anything.
I used to dumpster dive for old compsci books at the end of semesters. It's how I learned java, c, perl, vim, and SQL.
I'd always grab old books from school and work, but honestly they are horrible to learn from because things like java and c# have changed so much, you end up teaching yourself outdated stuff and then needing to relearn all of the new ways to do stuff. You're probably safe learning C from an older book though, as long as it's ansi c and not the original k&r book.