tl:dr; If you are a Linux/BSD user, this book will help you understand your system.
There are Stack Overflow answers, random blogs and tech articles. You can find man pages. What they lack is a comprehensive view of the full system, with historical background, a theme tying the topics together. They lack clarity, depth or accuracy. Nearly always, they lack humor.
UNIX and Linux System Administration Handbook is a very broad overview of all aspects of Unix system maintenance and upkeep: going from installation, package installation, and routine chores. It has in-depth coverage of critical sub-systems like networking and disks. These are covered over many chapters, devoted to hardware, services, best practices. There is deep coverage of user management, security issues for sites, authentication, logging. It has the obligatory chapter on shell programming, with bash & Python. There's information on config management tools like Ansible. It has much more than a system administrator would need through their first year. It is a large book and you will find yourself revisiting specific sections. It helps to keep an electronic copy for quick searches. The index and the table of contents are ideal for an admin with a paper copy.
The book gives you a superficial overview of virtualization, cloud systems, containers (Docker), and container orchestration tools like Kubernetes, Docker Swarm, Apache Mesos or Marathon. These sections are light: it gives you a background, why these systems exist and how to think about these systems. You can start a journey there if you want. These chapters were added later, and the coverage is satisfactory. Users need to have a rough idea of these, but each of those topics deserve a book-level treatment. The authors have kindly provided references to chase for the interested readers.
I've been a long-time Linux user, and I found the book's treatment of systemd level-headed and valuable. It provided a great preface with motivations for systemd, how it has evolved, and how best to use systemd to keep a system humming along and keep the system manageable.
The book was funny in all the right parts, without overdoing the humor or trying too hard.
It was published in 2017, but most of the sections are quite current. The book covers major Linux distributions (CentOS/RedHat and Debian/Ubuntu) and FreeBSD. The FreeBSD sections were great, especially alongside Linux so I had a Rosetta stone. The book's primary author, Evi Nemeth, is one of my favorite technical authors. Sadly, she passed away before this edition of the book was published, and I commend the current authors for carrying on her tradition, and their work on this edition.
In 2021, the diversity in Unix systems has shrunk incredibly. Solaris is history; FreeBSD is relegated to core adherents and the occasional technical Mac OS user. Cloud systems are gathering behind Linux instances. The ease of creating a Linux instance means that more users will find themselves responsible for a Linux machine. Unix used to mean SunOS: something formal, expensive. Something meriting a real education. Now, Linux powers everything from your phone to your Top500 supercomputer. A cloud instance can be yours for dollars a month. The power is still there, if you can invest time in learning it.
Veteran system administrators will enjoy filling gaps in their knowledge. The casual user will gain a deeper understanding of their system with this light, engaging, fun read.