As a bit of a followup to my linking to the Elegant Universe series earlier, I'm currently reading A Brief History of Time by Hawking. It's an excellent high-level introduction to relativity and quantum mechanics, and does as good a job as is probably possible to illustrate the gap between the two. But I'm finding it a little light on the mathematics.

I figured Greg's probably covered this ground before, so I am hereby soliciting suggestions for further reading in this area from him, or anyone else who's read stuff in this area.

On the other hand, this is a great book for what it offers. The way it explains the relationship of time and space as an expanding cone emanating from any event in the universe (say, a star exploding) is the best way I've seen to explain the subtle implications of general relativity.