There was a short story I read years ago.
A man stumbles across a strange library in a mist, or perhaps he got lost in the stacks of a library, I don't remember. But the library has all those books that could have been written that never were, e.g. plays that Shakespeare could have/would have written, but never did/got around to, things of that nature.
The man has one hour or maybe the rest of the evening to roam the stacks, after which the library vanishes forever. Ring any bells?