Then there's Battlestar Galactica (new one). Around 5,000 years ago, something happened and humans were evicted from a world and settled in a solar system with 12 inhabited planets (and the Earthlings).
Although fiction, both have some ancient history that is vague and not fleshed out, and more and more of that history is revealed as the story progresses.
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.