The Dragon Knight (Tor Fantasy)

Thrown into the alternate universe of 14th-century England, 20th-century mathematician Jim Eckert and his wife, Angie, have become Sir James, Baron de Bois de Malencontri, and Lady Angela. A bit of a shock, but they'd coped with it. In fact they were dealing with their new life's hazards and magical manifestations just fine. Until Jim is suddenly transformed into a dragon. None too pleased about this latest turn of events, Jim turns for help to his neighbour, the magician S. Carolinus, who takes him on as an apprentice. When Prince Edward is captured by the French at the Battle of Poitiers, Jim sets out with some companions, including Aaargh the wolf, to join the English forces. He ends up as chief pawn in a battle against the Dark Powers―with extraordinary consequences.