Far Shore of Time

Frederik Pohl tells tale of adventure and suspense in the next century when humanity finds itself on the front of a war spanning all space and time.
When government agent Dan Dannerman joined a team investigating strange signals from deep space, he had no idea that he was about to be dragged into the midst of an interstellar war. Held captive on an alien world, Dannerman suffers horrific torture at the hands of the Beloved Leaders, an alien race working to bring about the eschaton - the collapse of the entire universe. When he is finally liberated in a sneak attack by the Horch, the sworn enemies of the Beloved Leaders, Dannerman is ready to strike back - but at whom?
Trusting neither side, Dannerman must gain the trust of the Horch and somehow convince them to send him back to Earth on a mission to warn his fellow humans of the approaching conflict. Allying himself with these alien, enemies of humanity's enemies, he will return to an Earth stranger than he can possibly imagine, an Earth that already has two Dan Dannermans - an Earth under siege by the Beloved Leaders. ...