This book is the 4th in the "Swallows and Amazons" series, and is a great favorite of ours, especially in the winter as we too dream of sailing over an iced over lake searching for the North Pole!In this book, two new children are introduced, Dorthea and Dick Callum. Dorthea (or Dot) is a budding novelist and Dick is her scientifically inclined brother. They are staying for the Christmas holidays at the Lake District with the woman who took care of their mother when she was young. Their father is an archaeologist and has been joined by their mother at a dig site somewhere in the mideast.So the adventurers this time become explorers. The lake has iced completely over, and a race to find the North Pole is started. But not with sailing boats, but with sailing sleds instead! So which of the sets of children will win? Will Dot and Dick be able to come up with a viable sailing sled? What will happen to the Blackett's when Nancy comes down with a case of the mumps and must be quarantined?! And what happens when a mixup about signals sets the race off prematurely?!How disaster is narrowly averted and turned into an exciting conclusion, you'll just have to read for yourself!So far, I have enjoyed how the children are real children. They are not smarter than their parents or other adults (whom they call "natives"), but they are able to figure things out and carry on without daily oversight. They combine poems and literary and historical characters that they've read about into their stories with delightful fun. For example, when the houseboat owned by Uncle Jim becomes frozen in, they rename it the "Fram", after the ice-bound boat captained by Dr. Nansen in 1895!