The linked book by Pierce is pretty fantastic as well. <a href="http://www.cis.upenn.edu/~bcpierce/sf/" rel="nofollow">http://www.cis.upenn.edu/~bcpierce/sf/</a>
I've been working on a similar project for high school algebra. Mine isn't as far along, so it's been nice to see some validation on my ideas, as well as get some thoughts on how I would improve.