NTP Best Current Practices
The IETF has published a new version of NTP Best Current Practices documenting learned best practices on how to run NTP servers and clients.
If you know a little about the NTP protocol reading it will be a quick way to learn more about how it works “in the wild” (including on your own systems).
Some of it is specific to the “reference” ntpd implementation, but much of it will apply to other clients or servers, too.