Mathews J., Walker R. Mathematical Methods of Physics 2ed 1970

Textbook in PDF format

This well-known text treats a variety of essential topics, ranging in difficulty from simple differential equations to group theory. Physical intuition, rather than rigor, is used to develop mathematical facility, and the authors have kept the text at a level consistent with the needs and abilities of upper-division students. This book covers subjects which are often ignored in traditional texts; for example, statistics and the fitting of experimental data, dispersion relations and super-convergence relations and the group SU(3).
Cal Tech Lecture Notes.
The course from which this text evolved was originally based on lectures by Professor R. P. Feynman at Cornell University, Much of Chapter 16 grew out of fruitful conversations with Dr. Sidney Coleman