I read this book on the strength of it being brought to my attention by Amazon when I was ordering some of Philip Kerr's Bernie Gunther novels (the March Violets triple-set).
I have long been fascinated by this period of history - what made ostensibly intelligent and rational people embrace Nazism, either wholeheartedly or by simply tolerating the pervading power of the State machinery in everyday life?
Previous reviewers have noted the lack of set-piece action (though the story itself is taut and believable), but I think the events of this book probably better reflect the reality of the situation, in particular the inability to trust all but a select few with your true thoughts, and the fact that simple kind deeds could leave you in very real danger.
In fact, when compared to Kerr's entertaining Gunther mysteries, Downing's protagonist (John Russell) emerges the much more plausible character. Gunther - particularly by the time we reach The One From The Other and A Quiet Flame - starts to enjoy some outrageous coincidences and good fortune, quite apart from the oddity that he has survived the war despite being on first-name terms with Artur Nebe and Reinhard Heydrich (amongst others).
So in short, a worthwhile and credible read with a good feel for the times. If you want a more action packed but less plausible feel for the same time period, go with Gunther.