Good Music Theory Textbook for Piano Students?

[scriabinophile]

For use with my high-school age intermediate & advanced pianos students, I'm looking for a good music theory textbook with a workbook/exercises, and also with a key/answers.  I'd like something perhaps suited to college theory 101, for people with prior background in music but not in the study of music theory.... a book that is good and clear, but not childish or dumbed down. 
Any suggestions?