Once you sign on to the math train you will find there are innumerable nooks and crannies to dive into. There are purely theoretical approaches (theory of functions and others) and more directly practical approaches (perturbation methods and others). All find their occasional application in solving what would be an otherwise tough problem.

A book I have found useful when reminding myself of terrain that I haven't been over for a while is:

Handbook of Applied Mathematics, Edited by Carl E Pearson.

There are always two problems in using mathematics in engineering. The first is formidable - knowing and remembering that there is a tool available which has bearing on the problem at hand. This book is helps with this, and also aids in the second problem - actually using the tool which is usually a more straightforward problem of looking up and absorbing or reabsorbing the information on the method.

There is a third problem - recognizing that a given tool can apply to a given problem. That is usually beyond the province of us mere mortals. The guys who solve these third problems have names like Maxwell, Heaviside and Dirac.