Guns of the South is an excellent standalone novel. I also like the Southern Victory series...I especially think the Jake Featherstone novels are underrated as teaching materials as to what makes for the sort of horrific genocide that happened in Nazi Germany. I'd be interested in knowing whether the culmination of the Southern Victory novels were intended when he first wrote the initial ones in the series. He does a really excellent job in drawing the parallels.
One of my favorite Turtledove standalones is "Marching Through Peachtree". This is a challenge book, I think, in which the reader is intended not so much as to figure out the connections as to understand the connection between the two terms.
For example, Richmond is called Nonsuch.
Why?
In order to know why, you have to know that there was once in England a Pale of Richmond (the term Pale references the fact that, with Cornwall, Lancaster and Chester, is one of the few titles which is Palatine, and from which the earls of the same derive their title) and that in that Pale there was a palace of Nonsuch. How about General Guildenstern in place of Rosecrans? Well, how about the G&S musical Rosenkrantz and Guildenstern?
This book was just so much fun to read; a whimsical journey on a highway of references, just thoroughly, thoroughly enjoyable.
I've also read The World War series, about how war breaks out between Nazi Germany and the Allies...only to be stopped by an alien invasion and the consequent unified attempt to expel the aliens, along with the Vietnam reference as the invaders fighting the guerilla wars find themselves addicted to a substance--ginger--which undermines their war effort...
But next to the Southern Victory, specifically the Jake Featherstone, novels, and in way tied with them, is Turtledove's "Into the Darkness" series, his retelling of WWII where dragons take the place of divebombers and genocide occurs as the blood of a race is used to create magic. All the players are there, in so many different retellings, I really think that, as the fruit of his imagination, this is Turtledove's best work.
Very few authors have had as much to offer as Turtledove. Most flame out after one or two good books. Over his career, however, Turtledove has produced quality work after quality work. The "Into the Darkness" series kept me enthralled for a couple of years, eagerly awaiting the new installments, as did the Featherstone books. If he's coasting now, and yes I think he is, so what. He is one of the few authors that has been with me a lifetime and for whom I have nothing but admiration for the fun he's given me.
A well deserved wiki.
Jake Featherston - Harry Turtledove Wiki - Historical fiction, Days of Infamy, Homeward Bound