This is the last of the trilogy that they wrote together and I read the first two before I was on LJ. You can read the Amazon reviews for Sorcery and Cecelia or The Enchanted Chocolate Pot and The Grand Tour.
This book is again correspondance between four people and back then, it took a while to get from one place to another. This is a world where magic is common and one couple is sent off to find out what happened to a German magic surveyer. They find dastardly plots, see transformed magicians, and one couple ends up with a superfluous child*. It's fun to read and although it leaves room for a book about the twins of one couple (in another ten years), it looks like that's not happening. You definitely want to read the first two books before this one.
*They give readers a big clue about the extra kid partway through and the problem with this is that's info that everybody in the book already knows. They aren't usually this loose with problems like this, so I don't know what happened.