A friend of mine is looking for an old book.
He remembers it being called The Infinite Tower although he may be wrong, and the author being a woman.
It is set in a fantasy/D&D type world in which there is a tower that mirrors its interior to match the minds of those who are inside it.
Two kids enter the tower and do not come out. Two adults are hired to rescue them. When they enter the tower they find themselves in our world. Apparently, for the kids who live in the fantasy world, our world is their fantasy, and so ours is the world in which the rescuers find themselves.