The Man Who Folded Himself

The Man Who Folded Himself is a 1973 science fiction novel by David Gerrold that deals with time travel. It was nominated for the Nebula Award for Best Novel in 1973 and the Hugo Award for Best Novel in 1974. Pretty clearly Gerrold's attempt at writing the end-all and be-all time-travel looping novel, it apparently had another f-word in its title, but the publisher wouldn't go for it.

The story follows Daniel (in 1975), a college student, whose uncle increases his allowance (to help with living expenses) if he promises to keep a journal. After his uncle dies, Daniel inherits a "Timebelt" that allows him to travel in time so he goes into the future, meets his future self and makes a large winning on the horse races. Soon after he realises he has to accompany his younger self to the previous day at the horse races and make the same results.

Over the course of the story he learns more and more about the Timebelt and ponders how else he can help himself through the time travel...

This novel provides examples of:

 * All the Myriad Ways: Daniel gets a little unhinged over time, screwing with history just to see what would happen. At one point he stops himself from preventing the assassination of RFK after President Kennedy's efforts accidentally lead to a Coca Cola bottle being discovered in the ruins of Pompeii, thus providing public proof of time travel being possible -- as opposed to going back to pick up the bottle.
 * The Fifties: Daniel goes back to live through this entire era a few times, as he feels it's the best and most hopeful point in American history.
 * Future Me Scares Me: Those hollow-eyed old guys having orgies at their shared mansion creep Daniel out.
 * Population Control: Daniel speculates at one point that the Timebelt might have been originally invented as a humane way to dispose of excess humanity, as anyone who uses it to travel never reappears in their 'original' timeline but in a splinter universe. Thus thrill-seekers, sociopaths, or criminals become somebody else's problem.
 * Screw Yourself: A lot.
 * Temporal Paradox: Averted. Rather than simply traveling through a single timeline, history splits off wherever Daniel travels. Whatever moment in history he "leaves" keeps going on forever, with Daniel simply having vanished.
 * Time Travel for Fun And Profit: Daniel first uses his belt to cheat at horse racing. This would have bitten him in the ass, as winning too often draws the wrong sort of attention, but he is forewarned by his future self. After that, Daniel gets a bit more canny (and wanton) about abusing time travel for fun and profit.
 * Ripple Effect Proof Memory