This is absolutely one of the best "popular" science books I have ever read. It is not a list of scientists with their discoveries and inventions. The books is written like a novel. So, once you start to read, you can't really stop. The flow of events and the language the author use make you finish the book as soon as possible.
First of all, I think the primary objective of the author is to ask (and show) us how important ethics in science is. Are the scientists independent from their governments, sponsors or funders? How responsible is a scientist for the outcome of his/her invention or discovery? Should a scientist continue his/her research under cruel (even crazy) people?
What John Cornwell does in the book is that without being subjective, he just presents us the evidences and events as they occurred and usually in a chronological order. So, I think it is our duty to answer the questions above. I want to quote from the book here:
"...(of Wernher von Braun) that he did not care whether he worked for Uncle Joe or Uncle Sam: 'all I really wanted was an uncle who was rich.'"
Secondly, we see how developed science was in Germany in the first third of 20th century. It is clear how important the Jewish scientists and others that were expelled and ran away from Germany were to modern science. They basically changed the leadership of science and changed the balance of the WWII as well.
Even under the power of Nazis and the Third Reich, Germany did huge progress in science.
We can also see the greed of people. Nazi scientists tried to influence the government to get promotion, better positions or sometimes to have the government assign a certain person to a position. This is not special to that certain era. These people are still around us in every aspect of life, science and business.
The book is full of details about Hitler himself too. For example he wasn't fond of jets, atomic bomb and rockets at first. I can not imagine what would have happened if he was interested in these areas when he came to power. We learn the way he look at science and technology sometimes surprisingly stupid and sometimes surprisingly smart.
There were moments that scientists such as Max Planck, Heisenberg and several others opposed Hitler for his decision of dismissal of the Jews from civil service and science. Actually, this book helped me to learn about the role of Heisenberg (although still debated) in the building of an atomic bomb in Germany.
On the other hand, horrible things were done in concentration camps as "experimental science"and we all know that. I don't think anybody can support or defend this kind of science.
I can really write 10 pages about this book, but instead I strongly suggest that you should own the book and read it. The book literally tells us about hundreds of scientists from different disciplines (physics, chemistry, math, biology etc.). Fritz Haber, Otto Frisch, Niels Bohr, Ida and Walter Noddack, Lise Meitner just to name a few of the most famous ones. There are several other scientists that I haven't even heard their names.
The final thoughts and suggestions of the author is striking. He asks very important questions. Today, there is no Hitler. But, will the scientists oppose their "bosses" for the unpleasant use and outcomes of their research or will they obey their governments and funders? He suggests that scientists should come together and assemble small communities to determine what kind of actions they should take in these situations. He asks scientist to behave like Joesph Rotblat who resigned from Manhattan Project.