The author Clyde Robert Bulla comes up frequently in children's literature. He has written tons of books. I chose this one because we just finished our Middle Ages study:
The girls enjoyed it because it includes all the essential props of a Middle Ages story: a castle, knights, a sword, horses, lances, a dungeon, etc. There is a reasonable amount of adventure and suspense. I, however, did not like the book as much because it's not very creative.