The four Guthrie children, Alan, Dit, David, and Winkie, live in a rented apartment with their parents, and struggle to maintain the quiet their landlord and neighbors want. A new neighbor, John Leigh, understands their problem, and suggests there might be a home for the family in the small rural town he’s from. This idea meets with approval, and soon the Guthrie family is moving to a lovely country home on a large parcel of land. At the back of their new yard is a lake and a wide stream, and beyond that is forest. The Guthrie children have been camping before, and the idea of camping in the woods appeals to them. The family explores those woods and discovers they’re surrounded by two branches of another small stream, making it technically an island, though one invisible to the eye. The children beg to be allowed to camp on the island, and they’re given the chance. Tents and supplies are brought over on a raft they build, and soon they have a cosy camp. Exploring the island is one of their activities, and exciting discoveries are made, but strange things also happen suggesting someone else is also there, hidden from sight. There are hills to climb, a secret cave, a stone hut to build, shipwrecked sailors, and more appealing adventures.
The author’s full name was Clara Dean Marshall, and her books are enjoyable reading. I like this one from the endpaper map on. Recommended.