Top books on r/sysadmin

Learn Windows PowerShell in a Month of Lunches

Time Management for System Administrators

The Practice of System and Network Administration, Second Edition

UNIX and Linux System Administration Handbook, 4th Edition

Mastering Windows Server 2012 R2

The Practice of Cloud System Administration

Network Warrior

The Phoenix Project

DNS and BIND (5th Edition)

Exam Ref 70-410 Installing and Configuring Windows Server 2012 R2

Rtfm: Red Team Field Manual

MCSA Windows Server 2012 R2 Complete Study Guide

The Cuckoo's Egg

Windows Internals, Part 1

CompTIA Security+ Study Guide: SY0-401

Backup & Recovery

Windows Internals, Part 2

The Visible Ops Handbook