The Star Shard — Frederic S. Durbin

Twelve-year-old Cymbril is a slave on Thunder Rake, a gigantic wagon city that rolls from town to town carrying goods to be sold by its resident merchants. The Rake's master purchases a new slave, a mysterious boy named Loric who is one of the magical Fey. Because he can see in the dark, Loric's duty is to guide the Rake through the treacherous wilderness at night.Cymbril and Loric secretly join forces to plan their escape -- soon the two friends thread their way through a series of increasing dangers, encountering an enchanted market and deadly monsters as their one chance for freedom draws nearer.