D&D Companion: The Domain

The "Domain Game" has it's roots in Dave Arneson's original Blackmoor campaign, and was expanded upon in the D&D Companion Set.

Illustration by Erol Otus, from the World of Greyhawk folio (1980).

The Cook/Marsh Expert rulebook briefly covers taxes (10 gp per year or whatever the DM decides).  The Companion Set provides rules for populations and resources.

Additional rules involving administration (income, experience points, expenses, troops), tournaments, confidence level, and events* are also included.

*see also Arneson's "Chance Cards"

I think it's important to have rules for domain management, even though most of the time, these will merely serve as a backdrop to the main action in a campaign.