How Back-Back Got His Name by Thomas and Peter Weck, illustrated by Len DiSalvo

From chapters.ca:
Can you imagine what it would be like to lose you back!!? Well, that is exactly what happens to Plumton, the Opossum. Lima Bear and his clever friends become detectives searching for this missing back. Follow them as they try new and different ways of thinking to solve the mystery. See how they band together to protect each other in times of danger! Will they ever find Plumpton's back? The message of the story is: The tolerance of difference in others yields benefits. In the series entitled The Lima Bear Stories, each has a distinct personality that shines through in every story. While the children have no idea what twists and turns the story may take, they come to know the characters and have a pretty good idea of how they are likely to act in different situations and settings.