What Mad Universe
by Fredric Brown (1949)
The editor of a sci-fi pulp magazine is accidentally transported to a parallel universe where space travel is common, Earth is at war with creepy aliens, New York City isn’t safe after dark, and his girlfriend is with someone else. Regularly appears on “Greatest science fiction” lists.
Not sure why. At 65,600 words it’s a full novel length. The story swings from large info-dumps to crime, horror and sci-fi genres. Then, near the end comes a long explanation of multi-universe theory. And it ends with a thump and little explanation. I may try one of his crime novels, but the short story science fiction is definitely better.