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
Speaking of lizards and the civil war, there's a really weird set of books by William R Forstchen called The Lost Regiment series:
Rally Cry ,
Union Forever,
Terrible Swift Sword ,
Fateful Lightning,
Battle Hymn,
, A Band of Brothers ,
Men of War and
Never Sound Retreat. These are undoubtedly the strangest alternate world books ever written which deal with the civil war. A regiment of union soldiers is swept up into a vortex and taken to another dimension. There they find peoples of all earth's races and cultures : 16th Century Russia, ancient Carthage, Imperial Rome, Elizabeth's England, and many more.(Not the originals , of course...the descendants of those first brought, who simply reproduce their ancestors' mores.) These people were brought there by a race of giant, intelligent lizards (the Bantag), who regard humans as useful slaves and a food source. Unfortunately for them, each time they try to get more humans, the humans' technology and weapons is a little more advanced than the 'shipment' received before...Colonel Andrew Keane, a one armed veteran of many battles, decides to keep his band together, try to overthrow the lizard rulers, and get back home...somehow. Sounds ridiculous, but it's a lot of fun, watching the interaction between the union soldiers and their republican, democratic ideals and the very different ideals and values of the other earth peoples, not to mention the opinions of the Bantag, who are intelligent, cultured beings themselves.