The conceit of the book is that it is 1964 in an alternate world in which the Germans won WW2 by figuring out that the Brits had cracked their codes and by doing a bit better on the eastern front in 1942. They now dominate all of Europe. The US did beat Japan, and so, just like in the real 1964, there is a bipolar world wrapped up in a Cold War, but this time between the US and Germany.
At one level, this is a very conventional detective novel. The characters will be quite familiar even to readers like me who dip into the genre pretty rarely. The plot is not as predictable as the Publisher's Weekly review (available on the amazon page) makes it out to be, but it is not wildly unpredictable either.
What raises this book a bit above the average is that the author, a Cambridge-educated journalist, puts in a lot of effort to make his alternate history credible. Many of the documents that play a role in the story are real, and the true history is carefully followed up to 1942. The author also has done a good job of porting over lessons from the real Cold War, and the real history of the Soviet Union in shaping his imagine of German society in 1964. For example, the revolution has been overtaken by bureaucratization, just as it was in the Soviet Union, and the Germans are fighting a costly war in the Urals against what remains of Russia (assisted covertly by the US), much like the Soviet war in Afghanistan. Harris also does a good job of laying out the Berlin that would have emerged from the grand(iose) plans of Albert Speer.
Perhaps oddly, the thing I will longest remember from the book is the historical document that it excerpts in which the German ambassador to the UK reports on a conversation with Joseph Kennedy. Kennedy is the US president in 1964 in the book's alternate history and was, in fact, US ambassador to the UK in the late 1930s and the father of Ted, John and Robert Kennedy. In the document, Kennedy reveals himself to be both a general anti-semite and, more narrowly, a Nazi sympathizer.
Overall, this book is somewhat recommended if you are into both alternate history and detective stories, but probably not otherwise.