1. Birkhoff, G. and Rota, G.-C. Ordinary Differential Equations, 3rd ed., Wiley, 1978. The first two sections of this chapter closely follow their presentation.
2. Dennery, P. and Krzywicki, A. Mathematics for Physicists, Harper and Row, 1967.
3. Watson, G. A Treatise on the Theory of Bessel Functions, 2nd ed., Cambridge University Press, 1952. As the name suggests, the definitive text and reference on Bessel functions.