Jean Thesman

From the book:

I ran after Will, but he kept ahead of me, never stumbling or losing his balance. The water was ankle deep most of the way, sloshing against driftwood and boulders where seaweed was tangled now. Will was nearly out of sight, but I was afraid to call out to him.

Afraid of everything now. My brother and the creatures on the cliff and people in Gull Walk.

Afraid that I was not dreaming after all.

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Charlotte Thacker and her siblings have been sent to stay with family friends, the Wardens, at their hotel on the edge of Puget Sound. As they settle in, Charlotte realizes that something is very disturbing about Gull Walk, and it is affecting her adopted younger brother, Will. He doesn't seem to have a shadow anymore. He claims to be able to talk with birds and "Spirit Lights." And he is spending too much time with the hotel's teenage handyman, Jonah. Then Charlotte learns Jonah and the Warden's secret: the nearby forest is a Darkwood, one of earth's mythical places, home to griffins, unicorns, wyverns, and dragons, and it is in mortal danger. The only one who can help is Will, who is himself an otherworldly Fair One. Will he abandon his adopted human family to save the Darkwood?