Originally Posted by Bostonian
Not at the OP's price point or grade level, but people interested in inexpensive math and science books should sign up for the Dover Math & Science Club, which offers a 20% discount off of Dover's already low prices. Dover reprints out-of-print "classic" books.
I was going to suggest Dover Recreational Math books too. Then, I found that some titles are cheaper on amazon
http://www.amazon.com/Moscow-Puzzle...428957413&sr=8-1&keywords=moscow+puzzles