The House Between the Worlds is one of MZB's non-Darkover novels. It gets into psychology and figuring out what is really real.

Cameron Fenton works at Berkely studying parapsychic phenomena. He agrees to take part in studying a new drug which appears to improve psychic abilities. But for him it has a most unusual side effect - it sends him to another world as a 'tweenman.

The world is the home of the Alfar, who are much like the Fairie of myth. He helps to rescue Kerridis, the Lady of the Alfar, from the ironmen.

Fenton finds himself in a difficult position. He cannot help but believe that what he saw and participated is real, while his coworkers insist it must be a drug-induced fantasy. Even as he finds proof in the real world, they continue to disbelieve him and assume he is suffering some very unique side effects.

The Alfar are in danger from the ironfolk and their ally, Pentarn. Fenton must find a way to go to them through the Worldhouse, so he can be there in body rather than as a nearly helpless 'tweenman.

I do enjoy this story. It's not the best story I've every read, but it's one I can comfortably reread every few years.