Linux Bible (10th edition)
Christopher Negus
Linux iptables Pocket Reference
Gregor N. Purdy
Asterisk: The Definitive Guide
Jim Van Meggelen & Russell Bryant & Leif Madsen
Offensive Shellcode from Scratch
Rishalin Pillay
Linux Advanced Routing and Traffic Control HOWTO
Gregory Maxwell & Remco Mook & Martijn Oosterhout & Paul Schroeder & Jasper Spaans & Bert Hubert