What if! What a wonderful phrase, I like to read alternative history, for me it's usually Science Fiction but it has to written by someone who knows history and the events and forces that drive history. Harry Turtledove has made alternative history into an art form, he has written so many books and series of books it's difficult to keep track but my fav series by him is the WorldWar series. The split in history happens in during WW2 when aliens invade! This series eventually ends up being 9 books long ( I wish it had been 18 books! it was a great read! ) he is often considered the master of Alternate history fiction. Another very good alternative history authors is S.M. Stirling, one of his best books is "The Domination" a great alternate history that splits at the time of the US civil war.
I like both Authors, but for different books. S.M.Stirling has a two great series on the West Coast of the USA after some sort of 'EMP' Event happens, destroying everything electronic and changing the way gunpowder burns (so guns do not work right). It focuses on an area of Oregon and as the series progresses, expands to the East Coast. It explores what the world would be like after such an event, millions dead, a return to a Medieval society. Theres the good, the bad, and the ugly, and a truly gripping series with a touch of the magical. I haven't read much Turtledove, but greatly enjoyed his two-book series on the Japanese Occupation of Pearl Harbor, the books following the lives of several characters from locals on Pearl Harbor, Japanese-Americans, a Japanese Soldier, American Pilot, etc.
The lizards vs. Stalin, the Nazis, the US and the UK. I've read the series and found the first few books okay. However, as the series wore on the story somehow seemed to become paler for me. Turtledove did have a strong cast of characters and he used them well. He's good but for me, not good enough in my book to be called THE master of alternate history fiction. I consider him A master, not THE master.
Meh, I thought some of his earlier works were good, but his later stuff to be "pulp" novels, able to be rapidly turned out to get a quick buck. I've long since lost interest in his work. What really turned me off was the aliens invade during WW2, IMHO, that's when Turtledove "jumped the shark." For fantasy novels, I'll stick to Terry Pratchett