Hi! I would be interested in knowing of good apps as well. In the past I have used the online website http://www.teoria.com. It is a wonderful sight that has both music theory and ear training (something which most other theory learning sources I have found often don’t address). This site isn’t geared towards harpists specifically, and maybe is a little more geared towards piano; but I have actually found that slightly helpful for my harp students. They have an easier time visualizing half/whole steps, major/minor chords, etc. on a piano keyboard rather than on the strings of the harp.