Our hero is Richard Prince, a British police officer turned spy. This is the second in the series and the book starts after his return from the first mission inside Nazi Germany. There is a sub-plot relating to his missing son and some love interest with a Danish police officer current residing in a German concentration camp.
The focus of this book is the supply of chromite from Turkey to the Germans. In the book, the Turks deny supplying this mineral and Prince is sent out to Istanbul to get the evidence. Istanbul was a lively spot in WW2, with the rival intelligence services operating in the city, and the Turks attempting to maintain their neutrality. The atmosphere is well captured in the early chapters.
Prince is struggling to get any evidence and falls in with local Jewish criminal elements who in return for helping them, find the chrome shipping dock. This help involves a trip to occupied Greece with some close escapes from capture. On his return, he is taken to the dock where he takes photos. Needing more evidence, he is smuggled onto the ship and follows the chromite from Istanbul to Romania and then up the Danube before arriving at the Czech factory complex at Pilsen. He links up with the local resistance who help him to plan a route home.
I won't spoil the detail of the story, which gallops along and rarely leaves the reader bored.
The basic plot is based on fact. The giant Škoda factory in Pilsen was renamed Reichswerke Hermann Göring during the war and was turned by the Nazis into a major armaments’ manufacturer. Turkish chromite did end up there and was added to steel making it harder and less likely to rust.
It is certainly the case that the British wanted to stop the exports, but I am not sure it was much of a secret, or that the Turks denied the shipments. In October 1941, the Clodius Agreement was signed whereby Turkey agreed to export up 45,000 tons of chromite ore to Germany in 1941-1942, and 90,000 tons of the mineral in each of 1943 and 1944, contingent on Germany's supplies of military equipment to Turkey. The Germans were to provide as many as 117 railway locomotives and 1,250 freight rail cars to transport the ore. This still honoured agreements with Britain, so actually fell short of what the Germans wanted. They swallowed the deal given that Turkey was their major source of the mineral.
However, this is fiction and it makes a good story, very well told.