I just bought a Math Dictionary from Usbourne Books. My engineer DH looked it over and said, "Wow! They really got everything in here, didn't they?" It's got a paragraph of bare bones, no-frills explanation on every bit of math I've ever seen (through college calculus). It's the closest thing to a math bible that I've ever seen.
Ha, and it's *really* attractively produced. DS-then-4 picked it up in a bookshop recently and was entranced, had to be torn away - so much so that he's getting it shortly, even though he obviously won't understand much of it yet. I had a lovely big maths book that I didn't understand much of from about the same age, and it was a good thing :-)
