The One and Only Ivan by Katherine Applegate

Oh, this is such a good story- sad, but good. Ivan is a gorilla who was captured as a baby and raised by humans. He lives alone in his domain (cage) in a mall. He is easy going and has come to accept his life at the mall, until a baby elephant is purchased to bring in more customers. Ivan decides the elephant should have a different life and does something about it.

The story is told from his perspective. It's so sweet and touching. It's meant to open the reader's eyes to how humans treat animals and it does so in a very powerful way, I think.