The Witch Trade — Michael Molloy

"I thought witches were only in fairy tales," said Abby doubtfully.
"Oh, no." The captain shook his head. "There's always been witches -- good ones and bad ones. And, what's more, they've been battling each other forever."

Abby Clover lived a very normal life.

Until.

Until a boy named Spike washes ashore, unable to remember where he came from or why he speaks the language of the sea.

Until Abby discovers how to make herself invisible.

Until a stranger with mystifying powers and a hidden past comes to town and changes Abby Clover's normal life forever.

Now Abby and Spike must join the stranger, Captain Adam Starlight, on a perilous journey to battle the evil Night Witches -- an enemy darker and dirtier than they could ever have imagined.