the Rainbow Land of his and Jamie Thomson's Duelmaster series. The name of the Palayal River by the city of Godorno suggests Tekumel, but nothing else about the setting does. The Forest of Arden features in both, though whether it's Shakespeare's, the one by the Avon, or some third high-fantasy variant is not clear.
Mark's titles in the series were Green Blood (love that title) and Coils of Hate. They were the nearest to being interactive novels - rather stronger on the novel side than on the interactivity, to be honest. The characters and locations would make a superb role-playing sourcebook, which is one clue that Mark might have taken them from his notes for the Orb campaign. (Which was not, as I never tire of telling people, even remotely Oriental, despite the Way of the Tiger books.)
And these maps..! I challenge any role-player to look at Godorno and not want to spend a few evenings adventuring there. One of Mark's biggest literary influences is Fritz Leiber Jr, and there's more than a hint of Lankhmar to those streets and canals. It does no harm to have the maps beautifully rendered by Leo Hartas, too.