Skylark of Valeron — E.E. "Doc" Smith

As the mighty spaceship Skylark roamed the intergalactic spaceways, scientist Richard Seaton and his companions found a world of disembodied intelligences. A world of four dimensions where time was insanely distorted and matter obeyed no terrestrial laws - where three-dimensional human intellects had to fight hard to thwart malevolent invisible mentalities...