The Universe Between — Alan E. Nourse

Bob Benedict is one of the few scientists able to make contact with the invisible, dangerous world of The Thresholders and return - sane! For years he has tried to transport - and receive - matter by transmitting it through the mysterious, parallel Threshold. At first his efforts met only with failure and madness. But now The Thresholders have risen in fury. Somehow Bob Benedict must make one more trip into that land of peril and pacify them before they succeed in hurling his planet - piece by piece - into the oblivion of infinity.