Essential System Administration: Tools and Techniques for Linux and Unix Administration, 3rd Edition

pfSense Essentials: The Complete Reference to the pfSense Internet Gateway and Firewall

Sudo Mastery (It Mastery)

Introduction to the Command Line (Third Edition): A Fat-Free Guide to Linux, Unix, and BSD Commands (Fat-Free Technology Guides)

Practical UNIX and Internet Security: Securing Solaris, Mac OS X, Linux & Free BSD

OpenBSD Mastery: Filesystems (IT Mastery)

Designing BSD Rootkits: An Introduction to Kernel Hacking

The Design And Implementation Of The Freebsd Operating System

Ed Mastery: The Standard Unix Text Editor (IT Mastery)

The Best of Freebsd Basics