I can understand why some people didn't rate this book highly: there are a number of one- and two-star ratings. Hayward is aiming at two goals - to produce an engaging adventure story and to evoke a crucial moment in the development of our wartime intelligence. At first I thought I'd find the continual insertions of catchprhases irritating, phrases such as humdinger and 'kein glas bier'. But soon I found the style very engaging, and from my rating you'll see I enjoyed reading this as a story and as history.
First of all, I think this is a very good read. It's well paced, brings across the tension and adventure of espionage, and draws the characters vivdly. The use of dialogue and description bring scenes very much to life. Also it cross-references details nicely, so that you don't get lost, but in a way that maintains the pace of the narrative. As such, it becomes compelling, and I've had to force myself to leave it to get to bed, it's that absorbing.
Having said that, it's more than just an adventure story. It's well backed up with detail from primary and secondary sources. I did wonder about the lack of footnotes, but in the bibliography you find a URL which takes you to extensive footnotes. In this way there is back-up, but not in a way that would compromise the narrative flow. So there's a solid apparatus supporting this evocation of the risks and uncertainties facing British Intelligence as we approached and then embraced the Second World War.
Talking of which, the actual story that unfolds is strange and enlightening. I suppose it's natural that we think in terms of things working to some kind of routine, managed by overseeing bodies. But in the build-up to the War, everyone was feeling their way and trying to get the edge over the perceived enemy. And thus the first forays into wartime intelligence were tentative, exploratory and thus uncertain. This was most especially true about the Double Cross system of turning, or suborning, enemy agents. Hayward brings out all the costly lessons learned by MI5 in particular that were expensive, but useful if viewed as experiments.
This book is thus an easy and compelling read in itself, but also it has great significance for anyone interested in getting a taste of the nature of intelligence. It evokes the guesswork involved, however educated and inspired those guesses might be, in terms of the often unreliable human factor. It's well worth your time as an adventure story and an educational experience.