G. Arfken and H. Weber, Mathematical Methods for Physics, 6th Ed., Elsevier Academic Press, 2005; (calculus of variations, integral equations, integral transform).
D. G. Dudley, Mathematical Foundations for Electromagnetic Theory, IEEE Press, 1994; (Sturm-Liouville problems, spectral domain, Delta function, Green functions, boundary value problem, electromagnetic sources).
C. A. Balanis, Advanced Engineering Electromagnetics, 2nd Ed., John Wiley & Sons, 2012; (Green functions).
M. Sadiku, Numerical Techniques in Electromagnetics, 2nd Ed., CRC Press., 2000; (numerical methods).