This practical guide demonstrates the incredible power and flexibility of the Linux command line. It covers design and use philosophies, providing a foundation that allows you to get the most from the command line and become a better SysAdmin. Learn why things are done as they are in the Linux and Unix worlds and how the Linux Philosophy should be applied to your role as a SysAdmin.
I'm a big fan of time management or more specifically using the time we have to be as effective (not just efficient) as possible. This book had been recommended to the team as it tackles this subject specifically for SysAdmins.
Windows Admin Slack is a great drive-by resource for the Windows sysadmin. This team has 33 public channels in total that cover different areas of helpful content on Windows administration.