Top books on r/sysadmin

Time Management for System Administrators

Learn Windows PowerShell in a Month of Lunches

The Practice of System and Network Administration, Second Edition

The Phoenix Project

The Practice of System and Network Administration

RHCSA/RHCE Red Hat Linux Certification Study Guide, Seventh Edition

UNIX and Linux System Administration Handbook, 4th Edition

Mastering Windows Server 2012 R2

CCNA Routing and Switching Study Guide

Group Policy

MCSA 70-410 Cert Guide R2

Site Reliability Engineering

RHCSA/RHCE Red Hat Linux Certification Study Guide