Gordon R Dickson 
Dragon on the Border [EPUB ebook] 
The Dragon Cycle Book 3

In The Dragon on the Border, Sir James, the Dragon Knight, faces his most terrifying challenge — the Hollow Men, spirits of the dead in empty suits of armour. Their weapons are all too real, and a slain Hollow Man can be resurrected within two days. As long as one of their unholy number endures, no Hollow Man can ever truly die. It’s a battle that could test any dragon. Not to mention a knight. Or an American. Or all three in one!