John Brunner 
Ladder in the Sky [EPUB ebook] 

The man in black picked up something which had been leaning beside him. A ring perhaps two feet wide . . . yet when the man in black laid it down on the floor it was as large as he was tall.The light went out. A bluish glow now emanated from the ring, revealing Bryda’s face ghastly gray as she leaned forward, and Yarco’s also, set and serious, and the conjurer’s impassive.And within the ring, where moments before there had been the bare planks of the floor, a shape that moved, and opened eyes glowing like coals, and spoke.’What world is this?’ the awful voice inquired.(First published 1962)