I really like my Nutrition textbook, Understanding Nutrition, 6th Edition (they prob. have the 7th Edition out now) by E. N. Whitney and S. R. Rolfes. ISBN 0-314-01239-7. Comprehensive and readable. Mine cost $51.00 new but I bet at college bookstores you could find used ones much cheaper.
Other texts might even be better. I like to find used textbooks that someone highlighted! Those highlights are often helpful!