Peopleware, by DeMarco and Lister. Discusses critical people-side issues in development, a key concern of agile methods.
Agile Software Development, by Alistair Cockburn. Emphasizes the principles and theory underlying agile methods, with a special focus on communication.
Agile Software Development Ecosystems, by Jim Highsmith. Includes interesting interviews with several agile method founders, in addition to a survey of several methods and key principles.