SSH Mastery: OpenSSH, PuTTY, Tunnels, and Keys

TLS Mastery: Beastie Edition

The Linux Command Line, 2nd Edition: A Complete Introduction

Networking for System Administrators

The Linux Programming Interface: A Linux and UNIX System Programming Handbook

Ed Mastery

How Linux Works, 3rd Edition: What Every Superuser Should Know

Sudo Mastery: 2nd Edition

Learning the bash Shell: Unix Shell Programming, Edition 3

Relayd & Httpd Mastery

Easy Linux Device Driver, Second Edition: First Step Towards Device Driver Programming

Kali Linux Cookbook

Letters to ed(1): The FreeBSD Journal Letters column, years 1-3

Understanding the Linux Kernel: From I/O Ports to Process Management, Edition 3

FreeBSD Mastery: Specialty Filesystems

RHCSA Red Hat Enterprise Linux 9: Training and Exam Preparation Guide (EX200), Third Edition

PAM Mastery

sed and awk Pocket Reference: Text Processing with Regular Expressions, Edition 2

Tarsnap Mastery