Shadows on the Sea (Aladdin Historical Fiction)

1942.
The U.S. is at war with Germany. Fourteen-year-old Jill Winter's mother is traveling to Newfoundland and must pass through the treacherous North Atlantic, where German submarines -- U-boats -- stalk like wolves. Jill's father, a famous pop singer, is on tour, so Jill is sent to Winter Haven, Maine, to stay with Nana.
Quarry, a local boy, says that "gossip ain't never been so good," and Jill soon discovers he's right -- Winter Haven is full of secrets and rumors. It seems everyone has something to hide -- even Nana! Jill doesn't know whom to trust, and she's worried for her mother's safety. And things get even worse when she finds a wounded carrier pigeon with a coded message attached to its leg.
Jill is determined to get to the bottom of all these mysteries, but when she uncovers the biggest secret of all, she finds herself in grave danger -- and must run for her life!