Not sure if you're still active in this, but I ran across a couple math books recently that would fit the bill of your project:
Amazon.com: Mathematics for the Nonmathematician (Dover Books Explaining Science): Books: Morris Kline
- this one's a half textbook, half popular science book intended for, as the title suggests, non-mathematicians. Covers standard (yet still interesting, of course) topics. Might be worth a browse if it's in your library.
Ten Must Read Books about Mathematics
-otherwise, this blog post lists 10 highly readable popular math books. I've read most of them and they're all pretty good.