Gordon R Dickson 
Dragon, the Earl, and the Troll [EPUB ebook] 
The Dragon Cycle Book 5

‘Tis the season for treachery: The Earl of Somerset is throwing a lavish Christmas feast, a revel above all others to celebrate the holiday. But the Dark Powers have chosen this joyous time to launch a massive offensive. An army of land-hungry trolls has laid siege outside the Earl’s fortress, while a nest of venomous traitors waits to hatch its deadly plan from the inside. Now Sir James, the brave Dragon Knight, must fight a battle unlike any he has ever known — a battle no man, knight or dragon can win. Not even if he is all three in one.