Guns of the South is the best TL-191 book by far. The rest feel like they were ripped off a template. In the Presence of Mine Enemies is, by far, my favorite Turtledove book. Most of his early stuff, such as Ruled Britannia, is also fairly good.
Guns of the South in not a TL-191 book. It has a different POD entirely. I like the premise, but it buys into the Marble Man mythology of Robert E. Lee way too much for my taste.
However, both
Guns of the South and the TL-191 series are more believable that Harry Harrison's
Stars and Stripes trilogy.