I'm sorry if this is an inappropriate place to ask this, but I'm still trying to get the hang of this site. So my book is Thomas Engel and Philip Reid Physical Chemistry $3^{rd}$ ed. I'm on the Chapter 17: "Commuting and Non-commuting Operators and the Surprising Consequences of Entanglement
However, it is missing the Chapter on Perturbation Theory, which I was told should've been where chapter 18 is. Instead of Ch. 18, we have A Quantum Mechanical Model for the Vibration and Rotation of Molecules. Where are some good sources for Pertubation Theory that would have this?