Command Line Kung Fu: Bash Scripting Tricks, Linux Shell Programming ...
Jason Cannon
Hacking - The art of exploitation
Jon Erickson
Bash Idioms: Write Powerful, Flexible, Readable Shell Scripts
Carl Albing & J P Vossen
Pluggable Authentication Modules
Kenneth Geisshirt
Linux Advanced Routing and Traffic Control HOWTO
Gregory Maxwell & Remco Mook & Martijn Oosterhout & Paul Schroeder & Jasper Spaans & Bert Hubert