The Book of Regency (2000)

From the book's introduction:

The Cerilian map shows borders criss-crossing the continent's surface and strange names identifying the regions. Even the nearly unexplored lands to the north have been divided into realms, and those realms into provinces. The borders represent centuries of war and diplomacy, treaties and intrigue.

But they represent so much more as well.

Every border defines a realm, but each realm is also defined by its ruler - or lack thereof. These realm regents, as they can be called, rule the land of Cerilia - not just a guild holding or a magical source, not a holy temple or enforcers of the law - but the land itself. Tied to Cerilia by their birthrights and their bloodlines, they govern the continent's realms and people.
This, then, is the book for them - a tome of information directed at realm regents, the men and women who govern the land. They might be warriors, priests, wizards or thieves, but they influence the events of Cerilia and draw its everchanging map boundaries.

A realm regent probably has at least one law holding to his name, or he likely won't be a realm regent for long. because of this fact, The Book of Regency also discusses law domains and issues related to their rulership.