I've been slowly making my way through the Arthur Ransom Swallowdale series. Due to some recommendations, I decided to skip book three (Peter Duck) and went straight for the forth one, Winter Holiday. There are too many great things to say about this book. Out of the three that I've read of the series so far, I feel this one is the strongest. The mixing of new characters with morse code/semiphore, historical obsessions, and new adventures make for an exciting read that also inspired me. I don't want to spoil a very neat part of the book for anyone, so all I want to say is that the way he works in codes and the creativity of these children into the story is just amazing. I was researching semiphore and radio codes for weeks after I read this book.
If you want to introduce a friend or relative to the Swallowdale characters but fear the technical sailing talk in Swallows & Amazons is too much, you should try this book. You are sucked right into the history of these kids without repetition of the past (as in found in the Second book, Swallowdale) nor are the stories from the past book integral to the plot. This allows someone who is just introduced to this world to be enveloped into the exploits of these children without feeling they are missing anything. A highly recommended book, especially while it is cold out. You'll be dreaming up adventures of your own thanks to this ficticious group of children.