Worldwar: In The Balance
In 1994, Harry Turtledove decided to run with what can rationally and scientifically be called one of the most awesome fictional concepts ever: Aliens invade during World War II. The opening book, In The Balance, starts things off with a bang.
A group of lizard-aliens known only as “The Race” with juuust the right amount of technological balancing to make for a great story attack a humanity that’s stronger and more advanced than anticipated. While the issues Turtledove has with long series (pacing, repetition, etc…) appear even during this book, they’re not deal-breakers. And the weaknesses are more than made up for by the amazing first impression the book makes.
If you like alternate history, science fiction, World War II, or just strange concepts in general, this is worth checking out.