While they love their Pennsylvania home, Jack and Annie also relish the trips to other places and times they are able to take, thanks to their Magic Tree House. In this, the thirty-third book in the series--isn't that crazy?--they travel to the inappropriately-named Greenland, rescue a stranded narwhal, and meet a Viking boy named Erikson, son of Eric the Red. At first the boy, Leif, is reluctant to give up a chance to kill the narwhal for its horn, but Annie persuades him to help them free the narwhal. In this version of history, the youngsters give Leif a map of North America that he will later use for his own exploration of the area. The story moves quickly, if somewhat improbably, and readers can learn a little bit about narwhals and Viking culture from reading this account. Plus, it all takes place in a timely fashion with the siblings back home in time for supper. Fourth and fifth graders will enjoy this one as well as its nonfiction companion. The black and white illustrations add even more interest to the story, and I had to wonder how those youngsters would explain their arrival in a tree house rather than a boat.