Ethical Hacker’s Penetration Testing Guide
Samir Kumar Rakshit
How Linux Works, 3rd Edition
Brian Ward
Linux Advanced Routing and Traffic Control HOWTO
Gregory Maxwell & Remco Mook & Martijn Oosterhout & Paul Schroeder & Jasper Spaans & Bert Hubert
Asterisk: The Definitive Guide
Jim Van Meggelen & Russell Bryant & Leif Madsen
Pluggable Authentication Modules
Kenneth Geisshirt