One of the handiest concepts for my understanding the process of music therapy was best put forward by Martin Gardiner in Frames of Mind, which came out back in the 80's when I was just starting my private practice in San Antonio. The basic notion is that there are different kinds of intelligence. As I remember, he talks about eight varieties. Four that I remember him talking about are mathematical, verbal, spatial and musical.
For me the specifics weren't nearly as important as having a way to understand how we all come to music with different frames of mind and skill sets. I also think it's handy to realize that just as there are various types of intelligence in general, there are also various types of musical intelligence. Some people have a great sense of rhythm, others can play in tune easily, others memorize easily, others feel chord changes easily, and others have what I call "theory mind" and can tell you a chord they hear is a second inversion with a flatted fifth and an added thirteenth.
Jeff Smiley, in the BE method, talks about the "numbers game", saying that some students are going to naturally "get" trumpet playing, no matter the instructions given by the teacher. The mistake teachers can make is thinking that what they've told those students will work for all the rest. What he does is lay out for the rest of us all those things about embouchure that "natural" players feel automatically. What he does for trumpet playing is pretty much what I'm hoping to do for music making in general.