I found the man pages to be a pretty good read. Use `man -t systemctl` to produce a pdf version. There are a couple of files you want to read, e.g. systemd.se

I found the man pages to be a pretty good read. Use `man -t systemctl` to produce a pdf version. There are a couple of files you want to read, e.g. systemd.service, systemd.unit, systemd, deamon etc. But those are linked at the `SEE ALSO` section.