Random late night science fiction blogging
He is "the master of alternative history", to quote a blurb, which is sort of true. He has written some gems, both short stories ("The Road not Taken" should keep physicists awake at night), and some of his novels or composite novels (Between the Rivers, Rule Brittannia).
His characteristic output though is tri- or quadrology (or more...) riffing on some simple alternative. These range from good solid work (the Videssos series - several series redoing Byzantium and late Roman Empire history) to interesting but "not quite there" like the Worldwar series (Aliens invade in the middle of WWII; spunky humans fight back, with understandable exceptions...), to atrocious reworking of history (one on an upside-down US Civil War; another on WWII fought with magic, bleh). One of his very interesting recent works is a multiple series on a "what if the Confederacy "won" the Civil War? Following the American Empire through multiple subsequent wars as history increasingly diverges (current epoch has a fascist confederacy allied with fascist France, British and Japanese Empire attacking a socialist USA allied with Imperial Germany).
Good things - he knows history, and he can write flowing readable books.
Bad things - he's writing too many books too quickly, the history is rubber-stamped and the characters getting thinner.
Turtledove tends to "multiple viewpoint character" settings which evolve slowly (ie some die off). But to set the tone he uses little cliche phrases to identify which scene you're in and too set the tone. This becomes really tiresome when repeated too many times (why, yes, we know this character has pale skin and the Navy keeps posting him in the tropics. Ha ha. Oh, and here comes the doctor gone native in Quebec saying "tabernac" again. Ha ha ha. As a literary device this is way overused and he badly needs an editor who will actually edit.
But good read overall.