From the first post on this thread:
"I'm thinking about books that haven't the tiniest bit of magic or the supernatural or a single mythical beast, no time-travel, but which I (and many others) would identify as fantasy because it takes place in an alternate reality. Not alternate history, or anything identified as happening in our future, but a place with its own history, geography, culture(s), etc, and therefore a story that could not possibly happen in our world because ... well, there is no place or time for it to happen."
I recommended Rachel Maddux's The Green Kingdom. A little group of people, not all of whom really believe there's a "green kingdom," set out using a map that takes them perhaps from somewhere in the Midwest to somewhere in the Rockies. This may be happening some time after the Depression. They do find a place where, once every ten years, apparently through a natural process, mountains part and access to the hidden realm is briefly possible. They go through. I don't want to summarize what happens. Animals and plants that are found nowhere on earth live there, but there is no magic, etc. Conflicts between and within characters complicate things. Characterization is far more important in this book than in a lot of sf and fantasy. I don't know anything quite like this book, which took the author years to write; apparently this long novel was originally much longer. Maddux appeared a couple of times in The Magazine of Fantasy and Science Fiction and published some other books and short stories, and, posthumously, an autobiography written in her twenties.