Speaking of used/rare books... One of these days I'm going to get serious about picking up a 1910 Brittanica set. If you're interested in some ebooks that won't break the bank and are into science fiction, check out the Baen Free Library. They have a bunch of them available for free download in just about any format imaginable. Beware though, the evil bastards at Baen understand the concept of "the first one's free". The 1632 series is especially good, and it has expanded from a single novel into about 15 books and over 70 "gazettes" of short stories set in the 1632 universe. I also recommend anything by John Ringo. I'd read a grocery list if he put it together.
About 80% of the current sci-fi I read is printed by Baen. I enjoy their “Monster Hunter’s International” by Larry Correia too. Ryk E. Spoor is a good author if a bit predictable.