That is the second GCHQ puzzle book, published by penguin. Both volume 1 and 2 are very fun and I recommend them. https://www.amazon.co.uk/GCHQ-Puzzle-Book-II/dp/0241365430
I sense FLOTUS is happier than ever, so much happier now that she has A REAL MISSION.
Also SB, I was not sure about all this decode stuff until I saw this book at the book shoppe yesterday: