'The North of the Old Kingdom refers to the area geographically located north of the Clayr's Glacier and is, like Ancelstierre, another world "the Old Kingdom basically impinges on." Sabriel and Torrigan imply in Lirael that it is actually ruled by various tribes and warlords (confirmed in "To Hold the Bridge"). Both Charter and Free Magic work at the border with the North, and at least one of the tribes has a necromancer with Free Magic, though it isn't specifically stated whether the magic is native and/or works farther into the North proper.
None of the action of the three books released so far takes place in the North, though "To Hold the Bridge" takes place around the river gorge which is the border between the worlds. The North is not included on Mogget's Map.
The novella "To Hold the Bridge" takes place in (or near) the North, though little is shown about the area or the northern tribes.