The Wrong Sword by Ted Rabinowitz

SPFBO8

The year is 1190. The place is Paris. If you are a noble, life is fleeting but wonderful. If you are a commoner, it's just fleeting. And on the streets of Paris, an enterprising ex-monk is selling "magic" swords to gullible knights with the help of a decrepit Welshman and a giant, sullen blacksmith. Henry the Rat thinks he has the world finally figured out. But then he sells the wrong sword to the wrong prince, and everything goes wrong. Before he knows it, he has been shanghaied on a quest to find a mythical blade. Unfortunately, the mythical weapon turns out to be real. And that's when the fun begins.