I have been trying to find this book for years. I read it back in the late 70s or early 80s. I think the title was Tower of Tebuthra, or The Tower of Tebuthra, though I might have the last word wrong, it’s been so long. I can’t find any mention of it anywhere and have no idea who the author was.
The story involved a girl whose brother was lost (or abducted) in an alternate dimension that they got to through a television set in the attic. She follows him into the other world to try and get him back, and arrives at some place within the world wet, cold, and hungry. She’s bathed, clothed and fed at the order of the master of the place, who tries to seduce her into joining him. At some point she discovers that everything in his place is an illusion, including the bath, clothing and food he’s offered her. I don’t remember any of the story after that, except that she rejects his attempts to recruit her and continues to search for her brother.