by Karen Hesse
The Music of Dolphins (or Music of the Dolphins as I always misremember it) is better than I remember it being as a kid. The descriptive language is great, especially after rereading Island of the Blue Dolphins.
A pilot spots a girl on an uninhabited island and rescues her. Her caretakers soon learn that Mila has been living in the ocean with the dolphins. She wants to please her new family but desperately misses her dolphin family.
I read this so long ago, probably something like third grade, and my copy of it is now so beaten up and worn, but it remains one of my very favorite books. EVER. I reread it every so often, and it still keeps its spot of one of the best books I've ever read.