The Grand Tour by Patricia Wrede and Caroline Stevermer

Cecy and Kate are happily married to Thomas and James.  Both couples decide to do a "Grand Tour" for their honeymoon trip but they have not gone far before they get caught up in another magical mystery.  Someone is stealing ancient magical artifacts and Thomas gets recruited to try to discover what is going on.  Of course, the two brides are not to be left behind, so they all focus their considerable talents, magical and otherwise, to try to figure who is behind a plot that threatens all of Europe, before it is too late.

This is the sequel to Sorcery and Cecilia, and was pretty much like the first one.  I don't believe this one was written as a letter exchange like the first one was, but it could have been.  Chapters alternate between excerpts from Kate's deposition and Cecy's diary.  Wrede adds a lot of feminine fun to the book.  For example, when they all arrive at Paris, the men are worried because they need to do some investigating and are afraid their wives will be bored without them.  Hah! of course not.  What do two newly married women of good fortune do the first time they come to Paris? Shop, of course.  The final showdown is especially fun.  I already have the third book checked out.  (2006, 469 p.)