I have mixed feelings about that series. I really like the reaction of different real historical figures to the alien invasion, but the aliens themselves ("Lizards") are WAY too contrived. They are very different from humans in some very fundamental ways, yet their technology and worse, economy is far too similar. Turtledove's main premise - to drop a circa 1990's military power into middle of World War II, - is just not consistent with physics.

A simple example - after their very first air raid, Lizards' fighter-bombers accelerate right out of the atmosphere for a low orbital pick-up. No modern fighter can do that, and any one which could do so, would have absolutely nothing to fear from any WWII interceptor. When out of ammo, it could simply leap over them. Yet from that point on, there is no mention of this ability, and the obvious tactics.

By itself that would be just a minor oversight, but the books are full of such inconsistencies. And in the second series, after colonization fleet arrived, they got worse. Turtledove makes the Lizard society almost an antlike communism where it suits him, and into supply-and-demand capitalism where it does not. It's just a mess.