Hey–
I found this website with the Feynman Lectures on Physics and there is a great article on the ammonia maser which I think is possibly more explanatory than what is in our Basedevant and Dalibard book: http://jsomers.net/papers/feynman-lectures-on-physics/
I don’t know if anyone else thought that was a really random subject to cover, but in this paper they seem to explain why it isn’t.
Chelsea
Actually it looks like all of Volume 3 is really good for Quantum Mechanics. And there is a pdf on Tensors! I have not ever been able to find anything clear about that. That is also presented in our Basedevant and Dalibard book in a very vague way.