Gordon R Dickson 
Dragon and the Fair Maid of Kent 
The Dragon Cycle Book 9

One of the tales of Jim Eckert, the young mathematician who travels to a parallel medieval world only to be transformed into a large but none too bright dragon named Gorbash. Now the Dragon Knight must confront the three disasters that lie in wait for any visitor to the English Middle ages: war, plague and Plantagenets The Dragon Knight, faces three new challenges – a plague caused by an invasion of shape-changing goblins, the arrival of the quarrelsome and ambitious Plantagenet dynasty, and a full-scale human-versus-goblin war.