For people who are not aware, "functional harmony" means the extended system of tonal harmony that you hear in basically almost all music apart from a very few exceptions (eg free atonality, serialism etc but there aren't many compared to the very large amount of functional harmony out there).
It's "functional" because the chords in the harmony have the function of establishing key centres, moving towards and away from them, establishing transitions to remote key areas etc.
One of the best technical books about functional harmony is "Structural Functions of Harmony" by Arnold Schoenberg[1], which is much shorter than his "Harmonielehre" but really packed with interesting detail about key and chord relationships.
True. Almost all non-western music also has functional harmony though. The notable exceptions are things like Indian classical music and things like gamelan (which have static modal harmony).
@tonaljs/key gives you some specifically functional info, but the vast bulk of it as useful for non-functional analysis – though it does assume 12-tone equal temperament.
