How to Read Mathematics, by Shai Simonson and Fernando Gouvea. Excellent. I'll definitely be using this with my students. (There's one little problem. They say 'term', about half-way down, when they mean factor.)
Check this out, by Alison Blank: Math is not Linear. Her blog is great, too.
Cool graphic of a wave, made using Mathematica.
Detexify lets you draw online. You draw one symbol, and it'll tell you what you drew. If you'd like to start using LaTeX online, this will find symbols for you. You can also use a site called sitmo. Maria explains that here. A few people have raved about Detexify. I'm not sure why. Is it just too cool that it does character recognition? Or is it more powerful than sitmo?
Video: Bobby McFerrin on the Pentatonic scale. I looked up pentatonic on Wikipedia, and the business about hemitonic or anhemitonic was beyond me. But I liked seeing the connection with the Orff method in kid’s music education.
Off the math topic, but on the I'm-writing-a-book topic: Uncertain Principles pointed me to this post about settling in to write.