Books in Flat Stanley series

There's no place on Earth a flat kid can't go! Stanley's not the only flat one...When a flat skull is discovered in Africa, Stanley Lambchop decides to travel there with his brother, Arthur, and their father, George. Maybe studying the skull will give them clues to Stanley's flatness. But once in Africa, the Lambchops are in for more adventure than they bargained for. From lions to zebras to elephants, it's the safari of a lifetime!