This is one of Harris’s earlier novels, published in 1992. It was made into a film in 1994.
Harris has written several historical fiction novels but this one is what they call alternate historical fiction or speculative fiction. It is also a murder/mystery thriller. I can say that I enjoyed this novel just as much as his historical novels as this one could also be classified as crime fiction, my favourite genre. For this novel you just need to imagine what Europe would have been like if Hitler had won the war.
From the book cover:
April 1964. The naked body of an old man floats in a lake on the outskirts of Berlin. In one week it will be Adolf Hitler’s 75th birthday. A terrible conspiracy is starting to unravel. . .
Fatherland is set in a world that almost existed but never was; the Berlin that Hitler’s architect, Albert Speer, planned to build – the hub of a victorious Third Reich, extending from the Rhine to the Urals.
Among its 10 million citizens is Zavier March, investigator with Berlin’s criminal police, the Kripo. A brilliant loner, March is assigned to the case of the old man in the lake. His trail leads him to discoveries of wartime corruption, Swiss bank vaults, love, danger and – most terrifying of all – to the deadly black heart of the Nazi state. . .
An exciting novel with many twists and turns and one that reinforced my thankfulness that Hitler had not won the war.
Gippsland Granny 
It sounds like a nightmare scenario, and no doubt a gripping read.
I think I have enjoyed all the Robert Harris books I have read.
That’s a great recommendation. I haven’t read any, I’ll have to try one.