I enjoyed this and will read more from this author. The pace was good throughout and I took no time to finish it.
Thomas Hill is a bookseller. He is called into the king's service in Oxford during the Civil War, mainly because of the recommendation of his old tutor: he was a brilliant mathematician and cryptanalyst. The main plot is to do with the decryption of a message which uses a Vigenère cipher, thought unbreakable at the time. Readers may find the details tiresome at times, but the plot moves on at a pace, with many colourful characters.
I can forgive a blooper early in the book, where someone uses the binomial genus-species Latin name for plants. It was invented a hundred years later by Linnaeus! Ah! well!