We recommend to work through the following books. Note, that this choice is very subjective.
Pay special attention to the theory of adiabatic invariants.
Work through the theory of radiation especially carefully.
Pay special attention to §51 of this book. In this chapter Landau uses the Wiener-Hopf method.
Nice presentation on quantisation with constraints.
An excellent introduction to path integrals.
Technique of instanton calculus in Quantum Mechanics!
Look at the interesting applications of path integrals to polaron problem (Feynman variational principle). Try to apply it to Quantum Mechanics problems you solved already.
There are many books on QFT. The books listed here are my personal recommendation for additional reading to a standard selection.
Excellent book on QFT. It is hard to read, but once you are through the book, QFT will become your working tool (not only in particle physics!)
QFT in theory of critical phenomena. Explicit calculations up to four loops!