Eleven: Shadow and Light by J.J. Bende

Mila's adoptive parents have always told her she's special, but she soon learns that "special" is the understatement of the century. On the last day of school, Mila notices a glow coming from her skin that only she can see, and she spends most of the summer at the beach trying to figure out what it means. One summer afternoon at the beach, a mysterious old man gives her a watch. More than a simple timepiece, the watch ushers her into another realm where Mila learns she is gifted, courageous and more powerful than she ever imagined.