There was a sequel? I thought the other Civil War books he wrote were along a different path, not necessarily on the same timeline. But they aren't bad at all.
I thought the WWII stuff was better, somewhat because it's a little easier to miss anachronistic stuff when the book is more recently based, but also because it's a tad more sci-fi and later in his writing. I think he has improved as he has written more.
He also wrote one about Elizabethan England being subjected to Spanish rule, with Shakespeare as its main character. Probably his best--it's called "Ruled Britannia."
'Guns of the South' was a stand-alone novel. The series that begins with 'How Few Remain' is a separate look at an alternate ending to the war, and subsequent events. It does not involve time-travelling Afrikaaners with hundreds of thousands of AKs.
The point of divergence in the series is that Lee's orders for the Battle of Antietam are never lost- and the battle proceeds as Lee intended. The South succeeds in secession, and there is an uneasy peace across the North-South border. When the First World War breaks out in Europe, battle lines are drawn again between the North and the South. It's an interesting read.