Heroes of the Valley - Jonathan Stroud

Halli Sveinsson is stumpy and swarthy, with a quick mind and an aptitude for getting in trouble. Bored with everyday chores and shepherding, he enjoys playing practical jokes.
When he plays a trick on Ragnar of the House of Hakonsson, he sets in motion a chain of events that will forever alter his destiny.
He is forced to leave home and go on a hero's quest complete with highway
robbers, terrifying monsters, and a girl who may be as fearless as he is. Along the way he will discover the truth about the legends, his family, and himself.