The End of Eternity by Isaac Asimov

The best time-travel story since H.G. Wells's The Time Machine, by the Grand Master of science fiction, the story of Andrew Harlan, Technician and Eternal. Andrew Harlan's job is to range through past and present centuries monitoring and even altering Time's myriad cause-and-effect relationships. As a Technician with the Allwhen Council, he initiates Reality Changes that may affect the lives of as many as fifty billion people - and a million or more of them may be so drastically affected as to be considered new individulas. Above all, therefore, a Technician must be dispassionate. An emotional make-up is a distinct handicap. Then Harlan meets Noÿs and falls victim to a phenomenon older than Time itself - love. Years of self-discipline are cast aside as Harlan uses the awesome techniques of the Eternals to twist Time so that he and Noÿs might survive... together.