I recently finished “The Book of God and Physics” by Enrique Joven and enjoyed it. It’s a book about the Voynich Manuscript which is a 15th century manuscript written in code that has never been deciphered.
The book is written in the scavenger hunt style that I really enjoy, where one clue leads to the next, and so on.
There are all sorts of theories about who wrote the Voynich Manuscript, who owned it, what the coded messages and drawings might mean. On Wikipedia, you can find pictures from the manuscript. It’s currently housed at Yale University. I wish I’d known about it since we lived near New Haven several years ago and it would have been awesome to see it (if it’s on display).