V. Gorbachuk/M. Gorbachuk: Boundary value problems for operator differential equations. 
Kluwer 1991, 350p. HFL 240.

B. Levitan/I. Sargsjan: Sturm-Liouville and Dirac operators.
Kluwer 1991, 350p. DFL 240.
Highly recommended to students of spectral theory and of the theory of differential equations.

V. Marchenko: Sturm-Liouville oerators and applications.
BirkhŠuser 1986, 370p. DM 148.