Simon R. Green

Blue Moon Rising (Darkwood) Simon R. Green Reprint edition

Rupert didn't especially want to be a prince. And he certainly never asked to be the second son of a royal line that really didn't need a spare. So he was sent out to slay a dragon and prove himself-a quest straight out of legend. But he also discovered the kinds of things legends tend to leave out, as well as the usual demons, goblins, the dreaded Night Witch-and even worse terrors hidden in the shadows of Darkwood.  
Rupert did find a fiery dragon-and a beautiful princess to rescue. But the dragon turned out to be a better friend than anyone back at the castle, and with the evil of Darkwood spreading, Rupert was going to need all the friends he could get.