Harry Turtledove trained as a historian, and 'alternate history' novels are one of his specialties. This is the first book in a series that I liked a lot, but many didn't. If you're not prepared to take this series on its own terms, don't bother.
The premise is that in May, 1942, just as the human race is getting ready for some serious mutual bloodletting, invaders from another star show up. These are the Race, known to humans as 'the Lizards', a species that has developed VERY slowly and patiently, and has conquered two other inhabited worlds. Both the alien races the Lizards have previously encountered were much like them. Humans aren't.
The Lizards have only sublight travel, and last surveyed Earth during the Crusades. They expect humanity to have advanced as much in 800 years as they or the other conquered races would have -- that is, barely at all. So they deploy their supersonic fighters and tanks, ready to roll over knights on horseback, and run smack into armies that are outclassed in some respects, and fully competitive in others.
A big factor determining if you'll like this series is whether you're willing to buy into the premise. You could say that these books are highly artificial -- let the Race arrive in 1932 and they'd walk over humans, let them show up in 1952 and they'd be nuked before they got a foothold. Or you can say 'the Race' is carefully contructed. Turtledove obviously started with the idea of an invasion during WWII that would be neither a pushover nor doomed, and the Lizards' characteristics follow logically from that plot requirement. As it is, humanity and the Race are "In the Balance," just matched to each other. This delighted me, but it clearly annoyed some of the reviewers.
The other big factor in deciding whether you'd like this series is pace and scope. The story develops a bit slowly, and not everyone will like following dozens of characters scattered over the globe as they make their separate decisions to fight or cooperate with the invaders, and try to stay alive in a world turned upside down. Also, things won't be resolved quickly, any more than the real WWII was over in a few months. Again, this annoyed some, but left me with lots to look forward to.
The "World War" series develops over four books, till the humans and the Race reach temporary balance, and continues in the "Colonization" series, when the aliens main settlement fleet finally arrives. If you want everything settled in one novel, this is definitely not for you. If, like me, you enjoy following dozens of characters spread through years and continents, you'll probably like this series.
Turtledove's characters are mostly well drawn, and the plotting fairly tight. His knowledge of history shows, and I mostly believed his assessments of how Hitler, Stalin, Churchill and Roosevelt would have reacted to these events. There are some things I would have insisted on changing if I was his editor, but overall I liked all four of the "Worldwar" series, and eagerly await the third volume in the "Colonization" series.