I enjoyed reading the HookTheory[1] eBook and got a lot out of it. It illustrates its numerous examples using a neat colored notation visualizer, and does a great job explaining melody and functional harmony. It plays simple piano midi to illustrate concepts using popular songs. There were quizzes every few pages to test your knowledge, which I found fun. There are some insights to be found here based lots of data they've gathered from analyzing popular song chord progressions.

My only complaint is that the audio playback has annoying zero-crossing pops.

[1] https://www.hooktheory.com/music-theory-for-songwriting