What makes Turtledove great is that he is actually a history professor. His books are not parallel universe stuff. They are alternate history stuff. Things like, what if, after Gettysburg, South African Nazi racists from the 21st century came back and supplied Lee with 100,000 AK-47’s and training to use them. Thanks to his understanding of the history of the time, it can make for VERY interesting reading.
The biggest problem with his 20’th century version (space aliens attack earth in the middle of WWII) is that the books don’t stand on their own very well. Each one doesn’t have a crisp ending. You pretty much have to read the whole series. It is very much like a soap opera/cliff hanger.
I’ve read the Ringworld series, the Rama series and quite a few other Sci-fi books but by far, my favorite author is not even Sci-fi - Tom Clancy. All of them are pikers compared to him in regards to writing style. They do come up with fascinating universes, etc. though.
A lot of Star Wars books are the same way. On their own, usually (though becoming less so in recent years) they're actually quite decent. However, if you take them as a whole, not only is there a much grander scope, but the overall story becomes much richer.