The Art of Keeping Cool by Janet Taylor Lisle

In 1942, Robert and his cousin Elliot uncover long-hidden family secrets while staying in their grandparents' Rhode Island town, where they also become involved with a German artist who is suspected of being a spy. Soon after Robert arrives at his grandfather's home in Rhode Island in 1942, just after the U.S. enters World War II, he learns of a German painter living in the woods who raises the community's suspicions, and of the mystery his RAF pilot father left behind after fleeing his home.