The Sonobe Unit, designed by Mitsunobu Sonobe, is so common that it doesn't have to be published in any book. You can find the diagrams scattered all over the Internet like how Jejemasters are scattered around my neighborhood. (I feel sorry for my neighbors). Permission from him is still required in order to post diagrams on public, though.
Now how is this related to Christmas and it is number 7 on our countdown? SO MANY reasons! Christmas Balls, decorations for your Christmas tree, and many more. You can even build a polyhedron shaped like a pine tree from this unit! (I just don't know how...but it's possible, right?) You can use large units made from wrapping paper and since the models are hollow, you can use the units to wrap a gift for your friends, family, and anyone who is not your enemy...