Chapter 9 of Nouredine Zettili's Quantum Mechanics: Concepts and Applications covers approximation methods for stationary states, including perturbation theory, the variational method, and the WKB approximation. Online platforms such as Numerade offer video walkthroughs and step-by-step solutions for the chapter's exercises, addressing common topics like degenerate perturbation theory and ground state energy estimation.
A typical solved problem: Find the eigenvalues of $\hatL^2$ and $\hatL_z$ acting on spherical harmonics.