I studied this at the beginning of my graduate degree but have to review it for my graduate exam. If it's not clear I'm talking about the $\beta = \frac{it}{\hbar} $ turning the integral of your propagator into a "partition function" and the ensuing analysis of eigenvalues as poles in the complex plane. I remember this as being a particularly beautiful and powerful approach but one that was difficult to follow the details and keep an intuitive picture of what's going on.

I'd like to find books (or articles or websites) that have a good qualitative and quantitative explanation of what's going on without getting too bogged down with the mathematical details. Shorter is better but not at the expense of clarity.