I'm tutoring some high school kids right now, but I don't actually own any high-school level math books. We've just been working out of the books they use at school. But I'm getting very irritated with their books. One kid has this geometry book that makes me want to pull my hair out. It's trying so hard to be informal that the kids aren't even aware of what the subject of the chapter is. So I'd like to purchase some books on my own. They'd be a good investment for my tutoring, and ideally I'd like something that could challenge good students. Heck, if I could learn something from it, even better. I was looking at the Art of Problem Solving series, which is apparently intended for competition level math, but they have a full curriculum, so I figured it would have the basics as well as challenging problems. Has anyone else heard of this series? Any good? If not, what should I look at instead?