Re: resources, really Google was my biggest resource as I was learning stuff. One book I really loved was "Signals, Sound, and Sensation" by William Hartmann which connected signal processing to human auditory processing -- I learned a ton from that book. For music notation, "Behind Bars" by Elaine Gould, which was super useful while building VexFlow.
I just checked out pitchy.ninja, it's awesome! I've been struggling doing ear training and singing over the past decade. This will help!
As a side question, do you have any good resources or tips on creating music related applications?