Rating - 3: worth reading once (borrow it from a library)
This is one of those books that, if you are looking for that sort of thing, it seems like a fine example, but I would not recommend it in general. It does what it sets out to do.
This is a popular history of cryptography, with an emphasis on military intelligence.
To be clear on terms, "popular" here means "for a general audience." This is not a deep or technical book. Rather, it skims the surface and spends time on subjects that make the best stories.
Personally, I am not much one for popular history, or very much for popular works on any subject. They can be useful introductions, but they provide the illusion that you have learned something while giving you just enough knowledge to get by at a cocktail party. Given the breadth of human knowledge, maybe that is all that can be hoped for.
This book does have new things to teach you, and it explains them in a way that lets you walk in knowing nothing on the subject. I do not know enough about cryptology to say if the presentation is fair and accurate. It seems all right, but I have the feeling that important details and qualifications were smoothed over to maintain the flow of the writing. It's popular history.
I mentioned a military focus. The authors write popular military history, so there is quite a bit from the American Civil War and World War II. (I am looking forward to reading their How to Feed an Army sometime.) They are not limited to that, so there are also bits on reading hieroglyphics or how you too can get Little Orphan Annie's secret messages using your decoder ring, for just two cereal box tops.
As a random note, through the wonders of the internet, you can see the authors doing a bit of primary research. Here is one finding soldiers from the Korean War to interview.
If you read this book, you will not know enough to break military codes or anything, but it will give something on the subject, with an idea of where to get more. Maybe if you combine it with The Name of This Book is Secret, you will have enough to get by.