This is a history of the technological advances in Britain during World War I. As such the title is misleading. Although there is a chapter on code-breaking (a brief and unsophisticated treatment), virtually all of the rest of the book is about scientific and engineering advances. There’s nothing about spies other than a few anecdotes about successful disinformation.
The book is not badly written, but the author makes the typical historian’s mistake of spending way too much time and space on the biographies of the inventors and not enough on the technology, i.e. the actual subject matter. Since I am more interested in the spycraft and cryptology, I was quite disappointed in this book.