What books have made you question alternative realities?

I know this has been a huge thing in Hollywood in the past few years but I have seen writers write fiction books about it. I saw the last popular post about time travel and it also made me think of alternative realities as well… I would love to do some research or read some publications on some physicists to see if it kinda is possible. But anyway, what are your favorite alternative reality books, or what books have made you question the possibility?

by Izumbu