In the 70s and 80s, film novelizations were much more of A Thing than they are now. (Heck, TV shows got novelized: I remember one for Charles In Charge.) The late George Gipe, a novelist and screenwriter (he co-wrote the Steve Martin/Carl Reiner films Dead Men Don't Wear Plaid and The Man With Two Brains), did a handful — I love his version of Gremlins — including the novelization of Back to the Future.
And it is jaw-dropping. And it goes in a million directions the final film doesn't. (It opens with a nuclear explosion. Really.) It's inventive, sometimes weirdly so.
Ryan North, who created the webcomic Dinosaur Comics, found a copy of that novelization and chronicled his re-read of it. Page by page. Each "...huh?"-causing detail. Here it is, in chronological order.
I read a bit of North's blog, in early 2012 when he started posting it, but never got anywhere near finishing it. MAYBE I WANT TO FINISH IT. Maybe you want to at least try it.