Apparently he's doing two series : One has to do with a timetraveling South African arms salesman who sells 21st century technology to General Lee, enabling him to win the war. That's the series that starts with
Guns of the South. The other series, which ALSO deals with the south winning the civil war, is separate from the "Guns of the South " storyline, as it has the southern victory due to rational causes consistent with the true history of the civil war, rather than time travelling. That one starts with "How Few Remain". I've only read "Guns", but judging from the amazon.com reviews, the less'science fiction-y' alternate history books are better, if only because the concept of a SA man going back in time to protect apartheid is now dated.
BTW, Harry Harrison did an alternate history book in which Great Britain enters the civil war on the side of the south-supposedly. Actually, they wish to conquer the land lost to Andrew Jackson. The CSA and the USA join forces against the common foe. Here's the reviews for those books :Stars and Stripes Forever, and Stars and Stripes in Peril
I never saw a sequel to Guns. I thought Turtledove got back into writing the WWII series with the little lizards. And I read Harrison's books. Didn't like them too much.
The thing I like about Turtledove's series is that for at least the first few books he uses actual quotes from the historical figure. Not to give away too much, but interestingly enough after Lincoln is booted out of office in '64, he starts the American Socialist Party. Now THAT I find not only plausible but quite likely