At first, I was a bit hesitant about this website because when I try to learn music theory by myself, there’s a lot of unnecessary for most practical musicians. Their book of music theory for pop music is a great primer for many beginners and intermediate musicians. It’s fairly easy to understand the ‘rules’ of how chord progressions work, in relation to pop music. The downfall is the amount of content, for example it doesn’t go through 7th chords. For under $15 in the iTunes, it isn’t enough for me, but it gives a lot of examples if you’re “pop-literate” because the examples are playable within the book (from my iPad).
I must say that the cool interface for hook theory’s music editor is actually quite intuitive after reading the quite tutorial. It uses a relative scale, so creating accidentals or other notes other than the 7 (plus octaves above and below) aren’t possible for the melody. But what’s cool is that you can create separate harmony and melody with their divided play area and export your final song into a midi file. For being in Beta, it’s worth a check out. They also have over 1,300 songs archived in this format, though most of the songs aren’t the complete songs, just so you are aware.