Iron Dragons: Book 1 - The Saramond Quests by Derek Gilbert

Dragon pee really stinks. With that bit of cautionary advice, master dragonslayer Davian begins his tale. Davian is a Third Level Master of The Order, a group of men who alone have the power to battle the most fearsome creatures on all of Saramond. For thousands of years, the brothers of The Order have protected their world. Now something has upset the balance of power between man and dragon, and Davian must face what appears to be a dragon that cannot be seen. But Davian is losing his grip on reality — and the fate of the world rests with a stable hand, an underfed priest, and a gardener from beyond the stars.