I recommend this book primarily because it contains Lord Wimsey and Harriet's solution of a Playfair cipher.
Most readers will recall Sherlock Holmes' solution of the Dancing Men cipher (recounted in Conan Doyle's The Adventure of the Dancing Men) and Legrand's solution of Captain Kidd's cipher (recounted in Poe's The Gold Bug). Both of these are simple substitution ciphers, easily broken if one knows certain facts about the English language, such as the order of letter frequency (E, T, O, A, N . . .) The Playfair cipher, on the other hand, is an order of magnitude more difficult to solve. It is a digram cipher, using pairs of letters (there are 26x26=676 possible digrams) instead of individual letters to encrypt the message. Tables of digram frequencies are of little use in decrypting short messages. Other methods are required. The mechanics are explained in the text.
The Playfair cipher was used operationally in WWII and to this day remains unsolvable as a one-time, short message, unknown-keyword cipher, unless you can guess one of the plaintext words. Wimsey and Harriet were lucky that they were dealing with an amateur.
Sayer's audacious trump of Conan Doyle and Poe caught my attention. The rest of the book is, to put it mildly, well-plotted. There is evidence here that native British intelligence far exceeds what one finds in the colonies. No wonder Sayers is so popular.