I really like Alternative History, and Harry Turtledove is a bit of a legend for it. He's written loads of alternative history. I read The Guns of the South recently.
It was alright. The plot is simple (?). Some South African racists have a time machine and bring a whole pile of AK-47's back to the US Civil War and give the guns to the south. These new guns are much better than what the North had, so the South win the war. The South African time travelling racists were hoping to change the course of history in favour of racism.
The books is pretty well written and flows well, Turtledove is a good writer, and can spin a good yarn. He can also get you into the world & minds of the 4 or 5 central characters of the book. However the whole books is set around a 5 year period from when the S.A.'s come and to just after the war, which is a shame. The most interesting part of alternative history novels is finding out what the current world could be like. However in this novel Turtledove doesn't tell us what this new world could be like.
I also was less keen on the book since it deals with the US Civil War. "What if the south won?" and "What if Hitler won?" are the 2 most common forms of alternative history hypothesis, so it can be quite tiresome to read another rehash of the same idea.