I read a book as a kid (in the early 1980s) that I would like to read again. Here’s the problem… I don’t remember anything about it except the protagonist was a boy of about 12 or so, and he falls through a hole from the present time to either the past or an alternate universe.
There may have been unicorns involved at some point.
I believe a girl of about his age helps him to find a cave back to his own time.
The cover was of a boy in a jean jacket falling.
I think it was geared toward the 10-14 set, but I was pretty young, maybe four or five (I started reading very early).
If you like that one, you might check out everything else Alexander Key has written. His books are all remarkably similar. My personal favorite is The Preposterous Adventures of Swimmer, about a talking otter.
Oh, I read The Forgotton Door when I was in school! My copy did have a picture of the kid falling backwards. Don’t think there were unicorns, but all the ordinary wild animals were friendly and unafraid of people, in the place where the kid came from. I think the family he hitched up with eventually went back to his homeplace with him.