This book is meant to be Russell's finest work. In many ways it is very typical in style of 1950's science fiction. It concerns an alien who infiltrates a planet to pass as a local and undermine them so they lose in a war to the aliens. That is a common enough premises, but the wrinkle is that here the alien is a human who goes undercover amongst the aliens.
The biggest problem is the aliens themselves. There society is exactly the same in every way as Hitler's Germany and the aliens are virtually identical to humans and basically identical socially and psychologically. That makes them seem like the least alien aliens ever, and it would have worked better if they weren't alien at all. The book reminds me of Ian Fleming's Bond books in the way it treats espionage. That's particularly when it treats of organized crime. I do not personally believe that either Russell or Fleming had any first hand knowledge of criminal low lifes.
The book is amusing but not a masterpiece by any means. Its probably better than some works by more celebrated science fiction authors of the 50s, but its really only for people who like this time in Science Fiction.