When it comes to servers, there's no more popular OS than Linux and its family of distributions. No matter which company you work for these days, there's a good chance at least part of their infrastructure runs some flavor of Linux.
Because of this, there's never been a better time to get into Linux system administration and engineering (along with the tangentially related disciplines), though I would say that, of course... being a Linux sysadmin.
This book aims to be your reference and guide for certain common tasks in the Linux world, from the mundane and basic to the fun and complex, though anything can be complex if you try hard enough. I hope that, while reading it, you find something new (preferably a lot of new things) and maybe come across suggestions that you wouldn't have come across otherwise.
We're also going to get practical in our work (because just reading about things is boring), using virtual machines throughout to achieve our intended goals.