Ahhh yes you're looking for Harry Turtledove, the Wizard of IF
Even though I've only read some of his work I think he's an excellent author, his Worldwar series was a bit far fetched, but The Great War series and the book you're referring to, In the Presence of Mine Enemies (IIRC) were excellent. I'd like to read more of his work.
edit: I made a mistake, the book I referenced theorised about what if Hitler won WWII. It may be the same author though, as he may well have covered several timelines.