The Man Who Folded Himself

In 1975, Daniel Eakins, a young college student, is visited by his Uncle Jim. Uncle Jim offers to increase Daniel's monthly allowance for living expenses as long as Daniel promises to keep a diary. Shortly after, Uncle Jim dies, and Daniel inherits a 'Timebelt' from him that allows the wearer to travel through time. Daniel quickly learns how to use the Timebelt and makes a few short jumps into his own future. He meets an alternate version of himself, who accompanies him to a race-track where the pair make a fortune betting on horse-racing. The following day, Daniel realises that it is his turn to guide his younger self through the previous day at the races; through this and other events the time-travelling Daniel learns more about the belt, about the nature of the 'timestream', and about his personal identity.
Daniel develops an interest in history, and uses the Timebelt to experience many of the significant events of world history and to view the future. He also changes the past to give himself a vast financial empire. At the same time, he begins to understand more of his own character, becoming less introverted and more confident in having a companion (either his past or future self) whom he is able to completely understand and relate to. No longer limited to living in a linear stretch of time, he sets about trying to change the world to fit his own desires, and having more and more strange experiences with his alternate selves. His activities result in a series of time paradoxes, which are explained by the existence of multiple universes and multiple histories: the Timebelt does not actually move the user around a single timeline, but to a new alternate reality which is the result of all the changes that Daniel has previously made to that reality.
Daniel repeatedly encounters alternate versions of himself, ultimately having sex with himself and beginning a relationship with himself. He learns that the changes he has made to his timeline have erased all traces of his childhood and early life. Finding himself lonely and hoping to correct the situation, he jumps backwards in time, where he meets a female version of himself called Diane. He begins a relationship with Diane. Diane soon becomes pregnant and gives birth to a son. Shortly after, Daniel and Diane separate, and Daniel raises his son in 1950s America. As Daniel ages, he becomes more obsessed with returning to the relationship he had with Diane, and then with the thought of his own inevitable death. He spends much of his time at a house party set in 1999, enjoying the company of dozens of versions of himself at different ages.
Daniel eventually realises that he has now become his Uncle Jim and that his son is actually the young future version of himself who will go on to inherit the Timebelt, and that his life has 'come full circle'. He makes preparations for after his death to ensure that the young Daniel experiences the same events that he did when he was the same age. The book ends with the young Daniel, who has read the now-complete diary, having to decide whether he will use the Timebelt.