The Man Who Folded Himself (1973) by David Gerrold

For this tale of all-consuming and repetitive time travel, Gerrold creates a kind of self-enclosed universe composed of nearly just one individual, a person who interacts almost exclusively with different versions of himself. It eventually evolves into a heady sort of solipsism and devolves into an ultimate form of narcissism, a development perhaps predictable due to the godlike power imbued on the individual by the miraculous device of this story - the timebelt. It's a very simplistic device; Gerrold provides some details on its operation near the beginning, the details of its controls, but it's something that a kid might come up with, a magic device that can take the wearer easily to any point in time. This device is merely a means; it's the impact on the individual which forms the crux of the story. Gerrold also briefly reveals - not more than an aside - the godlike nature of the belt's power, mentioning how the main character can journey to far-off places and times to observe the secrets of the universe, the true origins of things we can only guess at. But, such acquirement of ultimate knowledge does does not imbue the wearer with the same sort of omnipotence. The individual remains a weak, uninspired human being.
In the plot, the main character is a college student named Daniel; we never know much about his family or friends - there's only a sudden visit by an "Uncle Jim," who helps him out monetarily and leaves him the timebelt in his will.  In using this belt, Daniel soon runs into other versions of himself - at first, himself from several days in the future and later much older versions of himself. It soon turns out that "Uncle Jim" was simply an older version of Daniel - or so it is assumed. Becoming very wealthy through such simple methods as betting at the race track (knowing what the winners will be), Daniel drifts into a hedonistic lifestyle tainted by a perverse touch of solipsism - he begins an affair with himself (i.e. with another version of Daniel). He even meets a female version of himself and they marry, and have a son. He also ends up spending a lot of time in one particular future time era in a mansion co-inhabited by many versions of himself. Things soon come full circle.  It's a short book - the softcover BenBella edition is 120 pages - and a quick read; it's compulsive in a way, re-readable, but not very encouraging of the human condition. However, the reader will probably spend some time trying to wrap his mind around the cosmic movements of the main character and, crucially, the paradox of where the timebelt came from. BoG's Score: 7 out of 10

Trivia That Folded itself: the concept of the timebelt here - that it was never created but exists eternally yet externally from time & space - was also the concept of a special watch in the film Somewhere in Time (1980), also forming an eternal time loop. Gerrold has written many sf novels but is best known for his contribution to Star Trek early in his career, the TV episode The Trouble With Tribbles (1967).