James Blish 
Quincunx of Time [EPUB ebook] 
A Haertel Scholium Book

Travel at light speeds is common, and Earth — the first in the universe to discover it — now reigns over many worlds. But in order to rule effectively, Captain Robin Weinbaum, head of Security, needs an extremely advanced communications system. He gets it with the Dirac transmitter, the first machine capable of sending instantaneous messages anywhere in the universe. The only problem is that someone called J. Shelby Stevens has a machine that can do the impossible — tap the transmissions before they are even made! It is Weinbaum’s job to find Stevens. And unless he does, time will be abolished and the universe closed to Earth forever . . .