Signal by Cynthia DeFelice
Owen and his father move to a new area at the beginning of summer. With dad pulling extra hours at work, Owen is left alone with his dog Josie to explore the surrounding area. During one of their daily walks in the woods, they discover a mysterious girl with dazzling green eyes named Campion. Campion needs Owen’s help in signaling her parents, and asks Owen to return to her home – a distant planet in outer space.
This juvenile novel is very well written. It isn’t until the very end that you find out if Campion is really from outer space, or if she is just delusional.