My mom recommended this book to me a few months ago, as it's based on a silverback gorilla who now resides in the Atlanta Zoo! Katherine Applegate tells the story of the one and only Ivan, who lives in a mall. He's one of a few zoo animals who does shows a couple of times a day; there's also Stella, an older elephant, and Bob, a stray dog who sleeps on his belly at night. The mall owner/zoo keeper brings a baby elephant named Ruby into the show one day, and her optimistic outlook on life kicks Ivan's ambition to get out of the mall big-top into high gear.
It's a juvenile fiction book, so the wording is pretty simple. I almost wrote 'the narrative is pretty simple,' but had to backtrack; I think this book has a lot of depth that isn't always present in books for younger audiences. Ivan and Stella have been treated pretty badly by humans, and Ruby is a baby whose spirit is in danger of being squashed by the mall owner. Readers will definitely develop a great amount of empathy for the animals represented in the book, and it offers several opportunities for discussion with kids. Also, it won the Newbery Award, so you know it's good!