Spectral theory of differential operators Book

Oooh, so close.
I wanted this book to be better than it is. It's a fine book, but I had, perhaps, unrealistic expectations. It introduces non-maths types to various mathematical concepts (chaos, infinity, etc.) in a way that is very accessible. But then it sort of dumps us there - "whee! wasn't that fun?!" I would have much preferred that it go a bit deeper. But it's probably quite good for those who have no interest in math to begin with.