I've found systemd to be quite well documented. Here's a few of the resources I frequent:

Systemd docs: https://www.freedesktop.org/software/systemd/man/index.html

List of directives: https://www.freedesktop.org/software/systemd/man/systemd.dir...

Unit-specific configuration: https://www.freedesktop.org/software/systemd/man/systemd.uni...

Service-specific configuration: https://www.freedesktop.org/software/systemd/man/systemd.ser...

Timer-specific configuration: https://www.freedesktop.org/software/systemd/man/systemd.tim...