E. Hinch: Perturbation methods. Cambridge UP 1991, 160p. $ 60.

M. Holmes: Introduction to perturbation methods.
Springer 1995, 340p. $39.

4083 Tosio Kato: Perturbation theory for linear operators. Springer 1966.

4144 Urs Kirchgraber/Eduard Stiefel: Methoden der analytischen Stšrungsrechnung und ihre 
Anwendungen. Teubner 1978.

P. Lagerstrom: Matched asymptotic expansions.
Springer 1988, 250p. DM 78.

R. Malley: Singular perturbation methods for ordinary differential equations. 
Springer 1991, 225p. DM 78.