This course is designed for teens who want to further develop their skills in music theory but have limited time to study it. It includes eight units devoted to the following subject areas -- rhythm, minor intervals, natural and harmonic minor scales, major and minor key signatures, musical terms and symbols, diminished triads, dominant seventh chords and inversions in major keys, and I-IV-V7-I chord progressions in major and minor keys. Each unit contains a succinct introduction to the subject matter, short drills for five days of the week and extra credit examples with further challenges. While the books are geared toward the piano player, any instrumentalist can get a great deal of worth out of these books. This is especially true for vocalists looking to increase their ability to sight-sing and understand transpositions and piano accompaniment.
- MusicGearReview

This is actually quite remarkable for spending 10 weeks per book at five minutes a day!
Jean Ritter, Progressions