The Dragon and the Gnarly King - Gordon R. Dickson

Eckert's daring exploits have earned him a title - Baron de Bois de Malencontri et Riveroak - and he has settled down to a peaceful life as a feudal lord, with his beloved Angela at his side. But a new peril endangers his enchanted realm as the King of the Gnarlies teams up with the Earl of Cumberland, Jim's longtime rival, to kidnap his adopted son, Robert.