When Thomas Small moves into what may or may not be a haunted house, nothing is as it appears. Behind the walls and under the floors are secret passages, and all the people in the neighborhood seem to wear masks and disguises. It's really hard to tell the good guys from the bad guys, and even the living from the dead. The characters learn that you have to look beyond appearances to get to know people. They also learn that disguises can be quite useful from time to time, like when running from slavery, or even when trying to walk in someone else's shoes.
The emphasis on disguise in the story argues that we have to look beneath appearances and first impressions to find out who people really are.