Harry Turtledove is an excellent writer of alternative history. That genre deals with a divergence from our own history, by changing one or more events, and then surmising what would follow. His recent "Ruled Brittania" concerns what England would be like after the success (instead of the failure) of the Spanish Armada, and eventual invasion of England by the Spaniards.
This book is not alternative history. Advance and Retreat, the third book in the "Detina" series, is Altered History. Turtledove takes real US Civil War history, maps it into a new fantasy world, and retells the story with magic instead of technology and monarchy replacing democracy. In Detina, South is our North, East is our West, and both people and places have names that are excuses for punnery. Thus, the Cumbersome River (instead of Cumberland) or Summer Mountain (which is really Spring Hill). Some of the names are easy to figure out (Peachtree = Georgia), some require knowledge of Latin, Greek or Hebrew (Parthenia = Virginia, King Avram = Abraham Lincoln), some are cutesy (Peterpaulandia = Maryland), others are completely baffling (New Eborac = New York, Dothan = Alabama).
Turtledove does some things well in this book. The story is engaging, the battle scenes are riveting, and the characters are fascinating (for the most part). Even knowing how the events will turn out, since it corresponds with the US Civil War in 1865, I never lost interest. Even when Turtledove tells us sixty times that Doubting George isn't ready to invade, or Bell used to be a mighty warrior before he lost an arm and a leg, I kept going.
But some things are done poorly. Turtledove loved the punning more than keeping his world consistent, and many of the names simply rang false. Some walked out of Masterpiece Theatre, like Duke Edward of Arlington and Ned of the Forest, others arrived from mysterious lands with odd tongues (Generals Hesmucet and Peegeetee), yet no mention was ever made of this linguistic clash. At least in Turtledove's "Darkness" series, which is a similar fantasy remapping of World War II, each of the countries has consistant people and place names within their own borders.
While deciphering the puns and anagrams can be fun, they should not get in the way of the story. Yet the names do clash, a continual reminder that this novel is simply a retelling of a different land, far away. And one of the important parts of the story does not map correctly, for Turtledove has created swarthy "Detinans" from across the Western Ocean, who have defeated and enslaved native "blonds." More blonds remain, on on the other side of the Great River (Mississippi) -- ah, you see the problem! He's amalgamated Africans and Native Americans into one people! This off-note jars in an otherwise faithful (though upside-down) retelling of American history.
Recommended for Turtledove fans and Civil War buffs. Others take your chances.