If you are more into the songwriting / application side of music theory, definit...

cnanders on April 17, 2015 | parent | context | favorite | on: Chord progressions of 25,000 songs

If you are more into the songwriting / application side of music theory, definitely check out "Hooktheory I" http://www.hooktheory.com/music-theory-for-songwriting, which is the music theory / songwriting book written by the creators of the API used in the OP.