Stellaluna by Janell Cannon

If you're looking for a kid's book that teaches good, important lessons in a non-obvious way, the classic Stellaluna by Janell Cannon is one great option.  Yes, it's about friendship and adaptation, but just as importantly, it also teaches compassion for those who are different from us, for those who are often misunderstood.  Bats are becoming more and more endangered, and they're unfairly stereotyped and associated with things like a fear of the dark, and vampires.  In fact, a single bat can eat between 2000 and 6000 mosquitoes in a single night, they re-seed forests and rainforests, and are the only flying mammal on earth.  The artwork is gorgeous, there are funny moments for children, and we all learn that we're more alike than we are different.