I also found Piston's book to be more than satisfactory. It's much better than the drivel that seems to have made its way into university theory classes everywhere. Alternatively, Hindemith's theory books take an interesting perspective, although they're definitely not for the beginner. Hindemith develops an interesting tonal system that, paradoxically, includes the atonal music of Schonberg as well as jazz, to his chagrin, and other more modern developments.

One of the nice parts of Piston's Harmony is that it makes for good bedside reading if you have a cursory knowledge of theory. You'll have many aha! moments for very little effort. Additionally, it's an excellent reference. I find myself consulting it from time to time.