Pluggable Authentication Modules: The Definitive Guide to PAM for Linux SysAdmins and C Developers: A comprehensive and practical guide to PAM for Linux: how modules work and how to implement them
Had to read it for university, and it barely achieves it's purpose. My main interest was on how to develop a module and it is the most shallow part, barely explains the process is an overstatement. The rest is interesting and an easy read, at least.