The Practice of Network Security Monitoring
Richard Bejtlich
Linux for Makers
Aaron Newcomb
Linux Basics for Hackers
OccupyTheWeb
How Linux Works: What Every Superuser Should Know
Brian Ward
The Book of PF: A No-Nonsense Guide to the OpenBSD Firewall
Peter N.M. Hansteen
The Book of GNS3: Build Virtual Network Labs Using Cisco, Juniper, ...
Jason C. Neumann
Absolute OpenBSD
Michael W. Lucas