Guilty by Definition by Susie Dent
This is a clever mystery for word lovers. Martha is a senior editor for the Clarendon English Dictionary in Oxford, England. A series of cryptic letters and postcards starts arriving at the dictionary offices. They turn out to be related to the disappearance of Martha's sister, Charlie, years earlier. Martha and her intrepid band of fellow editors decide to try to solve the riddles on the missives to find out what happened to Charlie. Along the way, we readers learn the meanings of a lot of colorful and obscure words such as zugzwang (the obligation to make a move, but every move is detrimental).