Asymptotic Expansions: Their Derivation and Interpretation, R.B. Dingle (Academic Press, 1973)

I could write a long review of this wonderful book.  But, instead you should read the review by his noted student Professor Sir Michael Berry.  As an added benefit of reading this review you can download a copy of this long out of print book.  If you are a physicist interested in wave propagation, this book is a book that you must have and use.