Linux iptables Pocket Reference (Pocket Reference (O'Reilly))
Understanding Linux Network Internals
SSH Mastery: OpenSSH, PuTTY, Tunnels and Keys (IT Mastery Book 1)
How Linux Works: What Every Superuser Should Know
The Linux Command Line: A Complete Introduction
Automate the Boring Stuff with Python: Practical Programming for Total Beginners
Mastering OpenVPN
DNS and BIND
Black Hat Python: Python Programming for Hackers and Pentesters