Biography; Young Adult Literature
Fun fact: There really was a gorilla named Ivan who lived in a mall for twenty-seven years after being captured in the wild as a cute little thing. Ivan's sister was captured, too, and she really did die, and after Ivan's capture, he really did live with humans in their home until he grew too big, at which point he was sent to a mall in Washington state. Three decades later there really was public outcry about his living conditions, at which point Ivan actually did move to the zoo where he spent the rest of the days. And Ivan, for the record, really was quite the painter.
Insofar as real Ivan's story matches up with our Ivan's story, The One and Only Ivan is biographical. Insofar as everything else in the book is the creation of Applegate's imagination, though, the book becomes more biography-ish than true, blue biography.
Young Adult Literature for All Ages
You certainly don't have to be either young or an adult to appreciate The One and Only Ivan, but it was written for a younger audience. Some of the growing up elements in it—particularly in Ruby and Ivan's journeys—are appealing because they explore issues of youth, maturity, and overcoming our past, which is something each and everyone of us has to do in some way, at some point.