The Hungry Coat by Demi is one of my favorite books for teaching anyone (children/young adults) about how to treat others. In this story, a beloved townsman, Nasrettin is invited to a party with many other influential people. However, along the way his clothes get dirty as he helps out others. When he finally arrives at the party, the other guests (and host!) don't welcome Nasrettin as they usually do. This is a great opportunity to discuss why we choose people to be our friends, is it really what is on the inside that's important, and how others in the group can influence us.
Demi's books are always superb. The Empty Pot is another of my favorites.