1. B. M. Levitan, Inverse Sturm-Liouville Problems (Nauka, Moscow, 1984; VSP, Zeist, 1987).
2. B. M. Levitan and I. S. Sargsjan [Sargsyan], Sturm-Liouville and Dirac Operators (Nauka, Moscow, 1988; Kluwer Academic Publishers, Dordrecht, 1991).
3. V. A. Marchenko, Sturm-Liouville Operators and [Their] Applications (Naukova Dumka, Kiev, 1977; Birkhäuser, Basel-Boston, 1986).
4. G. Freiling and V. A. Yurko, Inverse Sturm-Liouville Problems and Their Applications (NOVA Science Publishers, Huntington, NY, 2001).
5. I. M. Gelfand and B. M. Levitan, “On the Determination of a Differential Equation from Its Spectral Function,” Izv. Akad. Nauk SSSR, Ser. Mat. 15, 309–360 (1951) [Amer. Math. Soc. Transl. (2) 1 (1955), 253–304].