Box Turtle - Venessa Roeder

Terrance the turtle was born without a shell, so he uses a cardboard box instead. Terrance loves his box. It keeps him dry on soggy days, safe from snooping strangers, and is big enough to cozy up with a friend. But when another turtle points out that Terrance's shell is, well, weird, he begins to wonder whether there might be a better shell out there...

Eventually, and through much trial and error, Terrance learns that there's nothing wrong with being different - especially when it comes to being yourself.