What's the Name of That Book??? discussion

Hello everyone! I read this book in the early 90's. It is a children's fantasy book. The only thing I can remember is a part where a boy is climbing a tree and someone tells him not to look down or he will be stuck in the tree forever.
I also have less than a reliable memory about the book cover having a princess on the cover. The boy may have entered the fantasy world through a mirror?
Not a lot of information, I know. Any help would be greatly appreciated!

This cover immediately leaped to mind, but I haven't actually read the book, and I may be misremembering the cover since it's hard to see the thumbnails!

This may be entirely unhelpful, but that sounds like something that could be in The Big Jump by Benjamin Elkin, but I haven't been able to confirm this

The Big Jump and Other Stories by Benjamin Elkin for Nick's suggestion.

Matt, are you still looking for this or did you find it?

No response, moving to Abandoned folder.

Matt (OP) was last active on the site in June 2016.