Definitely _Mathematics: A Human Endeavor_ by Harold R. Jacobs for me. I read it as part of a ~World of Math~ course way back in Junior High and I fell in love with math as a result of reading it. It whisked me away from the antiseptic world of rote arithmetic and it opened my eyes to how fun and exciting math can be. As a kid who reveled in puzzles of all types this was an excellent gateway to the world of problem solving.