A series of 4 books by Jean Cullop published by Scripture Union. The books feature two children, Luke and Rosie, who find themselves transported to the Island of Karensa where they encounter a series of adventures. The books are quite strongly allegorical, and the basic premise is somewhat similar to The Chronicles of Narnia. There are no talking beasts in Karensa though - all the characters are human, though there are some magical themes. All the books are quite short (about 140 pages).