Let's clarify something: despite its name, this book is not written by the same guy who wrote "The Inner Game of Golf." I'd heard great things about that book, and decided to pick up this one instead, since I care more about music than golf.
Huge mistake.
Green spends most this book paraphrasing Gallwey, but doing it in a way that comes off as sanctimonious and without adding anything of his own. As a result, TIGoM is like twice as long as TIGoG but somehow still manages to say nothing.
I'll save you some time with what this inner game stuff is all about:
1) do it for fun,
2) be aware of your performance,
3) the first step to fixing problems is to identify exactly what the problem is,
4) don't overthink it.
Good advice, but not good enough to warrant trudging through 242 pages of shit.