I was searching for in-depth music theory books for my piano students and compared several methods, and Clefi Music Notebooks is everything I was looking for. They put theory in practice right away through drawings, singing, listening and rhythm. They are very colorful and fun. Really nice artwork and well written - I would recommend these books to kids who learn any instruments. Their explanation of the Circle of Fifth in Level 2 is so colorful and fun, and it shows the logic behind it right away. Level 1 includes intervals, and Levels 3 and 4 - chords with harmonic functions, as well as tasks to harmonize a melody. I also like that the material does not look overwhelming or difficult - it will not take too much time to do those tasks in the lesson and then continue at home. These books will help any music student to understand logic in music, recognize patterns, analyze, compose, memorize, hear chord progressions, so that music becomes their native language they speak fluently. 5 stars!