There is an excellent standard text for music theory: Alfred's Essentials of Music Theory Complete. "Complete" is the right one to buy, spiral bound with all three books of the course. You can buy it on Amazon along with the Teacher's Answer Key. It is very well done. Each chapter has a page on a point of music theory followed by exercises where you write answers. It also has ear training exercises on the included CD. It is instrument independent, but so is music theory. You can cover the course gradually. 15 minutes a day will get you to a high level in about 2 or 3 months.