I'm only guessing, but I would have assumed that much of the content was about choice of chords. Which isn't going to be very different on concertina or autoharp (or guitar or piano or mandolin or...). I haven't seen any harmonic effect on your videos that couldn't be largely reproduced on other instruments.

Okay, maybe you don't bother discussing how to select and voice the right chord. In which case it's going to be a rather odd book.