I don't know what it is, but the concept of these books (series of four) seems cheesy on the surface (Aliens invade during the early years of WWII), but is amazingly engaging stuff. A great mix of characters and subplots and an objective view of all parties involved, even the aliens. Besides a good tale, the author (HArry Turtledove) does a great job of getting into the mindsets of the characters, which are all appropriate for the time and specific cultural context. I recommend them, especially if you are in need of a distraction.