Measurement by Paul Lockhart, a book on geometry for those who have little background in mathematics. I have not actually read it, but I have heard it hyped quite a bit by people who know what they're talking about, and I'm quite prepared to believe that it is excellent given that it was written by the author of A Mathematician's Lament. If you are already a math expert, you might not get much out of this, but if you want to become a math expert, I recommend it.
This is the monthly thread for posting media of various types that you've found that you enjoy. I find that exposure to LW ideas makes me less likely to enjoy some entertainment media that is otherwise quite popular, and finding media recommended by LWers is a good way to mitigate this. Post what you're reading, listening to, watching, and your opinion of it. Post recommendations to blogs. Post whatever media you feel like discussing! To see previous recommendations, check out the older threads.
Rules: