Similar to the previous chapter, I strongly suggest you visit Chapter 11, Troubleshooting and Monitoring Your Workloads to read about logging in Linux, because often the systemctl status command doesn't provide you with enough information. I also already pointed to the blog by Lennart Poettering, and the systemd website.
For Linux security in general, you can start reading the book Mastering Linux Security and Hardening by Donald A. Tevault. Many of the topics covered in this chapter and many more are explained with a great eye for detail.
The firewalld daemon has a project website, https://firewalld.org, with a blog and excellent documentation. For older distributions, the Wiki of ArchLinux is a good start to learn more: https://wiki.archlinux.org/index.php/iptables. And since iptables is utilized by firewalld, it's a good start before diving into...
