I definitely enjoyed Gettysburg Redux by John R Stuart
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Is that one really good? The description on Amazon doesn't inspire confidence.
The Gingrich & Forstchen one is very good, if it has any faults its that the last book of the trilogy has a bit of a problem with the writers having to write themselves out of the corner they have written themselves into.
(There's a bit of a rush to the conclusion, and the eventual outcome is at least as contrived as any alternative)
The last book is a downgrade from the others. It feels like they were afraid to have the Confederates win the war (even though they spent two books setting up doing just that), that they just did whatever to have them lose.
But it could be a little worse, it could be "The Day After Gettysburg" by Robert Conroy. He was a great alternate history author, my favorite easily, but that one was a serous mistep