Hyperspace — Patricia Fanthorpe

"Elpowa" industries the mammoth electro-engineering combine controlling half the galaxy, discovered the Threshold ... a scientific gateway to the fourth dimension. The industry and culture of a thousand planets would be revolutionised by that discovery. It would mean the end of orthodox space transport... vast fortunes were at stake... and a galactic war was in progress. Was it the spacemen who were trying to sabotage the "Elpowa" process? Who were the Others with their weird, super-human powers? Where did they come from, and what did they want? How could space liners and even whole cities disappear without trace?