Here is my list of top 5 books that every sysadmin should read.

UNIX and Linux System Administration Handbook (5th Edition)

The best books on the topic is absolutely this one:
UNIX and Linux System Administration Handbook (5th Edition)
This is an excellent book that is comprehensive and readable. The book covers RHEL 7.1, Ubuntu 17.04, Debian 9.0 and FreeBSD 11.0. Must read. You will not regret buying it.

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

The Linux Command Line, 2nd Edition: A Complete Introduction
You wont be disappointed, if you're a beginner, hobbyist, or looking to learn something new, you wont regret this purchase. The book is a perfect balance of essential and complex.

Absolute FreeBSD, 3rd Edition

Absolute FreeBSD, 3rd Edition: The Complete Guide to FreeBSD
Content rich and well written book. Even if you are not touching FreeBSD on your day to day routines, you will benefit a lot by reading this book. In my humble opinion every Linux sysadmin should know thing or two about awesome FreeBSD opereating system.

Securing DevOps

Securing DevOps
Every admin will benefit from this book. The author covers a whole pile of security topcis that you can apply in the DevOps field form TLS/SSL, protecting cloud infrastructure, protecting web applications...

Docker in Action 2nd Edition

Docker in Action 2nd Edition
This one is simple. You must read a book on containers technology. And this one covers Docker. Containers are the future of shipping and installing web applications.