For some reason, I have Sealion in my mind. Back in the 1950s, C.S. Forrester, of AFRICAN QUEEN and HORATIO HORNBLOWER fame, wrote a longish short story on Sealion, I'll let you guess how he has it turn out. .
There was also a serious alternative history of Sealion, called OPERATION SEALION that came out in the 1970s. Sorry I can't remember the author, dammit! It was interesting because it was written like a Cornelius Ryan WW2 history, complete with fake first-hand accounts and documentation. I read it a long time ago and am trying to find it again.
Not a novel, but extremely interesting is British historian Niall Fergusson's discussion of the "What-ifs of the First World War and how they could have changed what happened afterwards--see the last chapter of THE PITY OF WAR.
There's also the alt-history "What-If" series, which is global in scope, but has some interesting articles about WW2