Good sources to understand Stat Mech with concepts I never had a proper course on Statistical Mechanics during my undergrad. I've tried picking up the subject by reading books, but I find that many focus on the mathematical derivations and I get easily lost in the details.

I was wondering if someone could point me to some useful reference materials where I could better understand concepts that are confusing to me. For example, why do chemical systems overcome energy barriers? I know that when a molecule samples a potential energy surface, the different states are occupied with probabilities relating to their energy difference according to the Boltzmann law. But, in classical physics, a ball rolling down a hill would eventually get stuck in the energy minima? I fail to see where do chemical systems get the energy from to be constantly in motion.

Sorry if this is a vague question or ill-posed, but as you can see I have some confusion of concepts in my mind which is not allowing me to develop a more accurate question.
I highly recommend Molecular Driving Forces by Dill &amp; Bromberg. 

https://books.google.com/books/about/Molecular_Driving_Forces.html?id=hdeODhjp1bUC

Contains many examples throughout to help with conceptual and mathematical understanding.