It is true that reads like a novel. We saw the constant struggle between code creators against the rioters code, each repeater ingenuity to take over the other. Every major advance in the history of cryptography is illustrated by an important historical event: From the decipherment of hieroglyphs using the Rosetta Stone, the German Enigma machine via RSA, which provides privacy on the internet. What is strong is that it seems to live those magical moments that were true turning points in our history.
So who is it for? Well for all! No need to be a math student to understand this book. Far from it (well except maybe for the very end of the book on quantum mechanics and yet it is only to understand the examples), I would say the contrary a literary mind as most of the decryption techniques involve linguistics.
Secret codes ... Far from my cup of tea. It made me want to buy it was so enthusiastic feedback from users. My turn to leave mine there and encourage others to do the same!