Bleistein, N. and Handelsman, R.A. (1986) Asymptotic expansions of integrals. New York: Dover Publications.
de Bruijn, M.G. (1981) Asymptotic methods in analysis. 3rd ed. New York: Dover.
Simmonds, J.G. and Mann, J.E. (1998) A first look at perturbation theory. 2nd ed. Mineola, N.Y.: Dover Publications.
Tricomi, F.G. (2012) Integral Equations. Dover Publications.