The universe as a roundabout: William Sleator, The Green Futures of Tycho. New York: Tor, 2006. First published in 1981.
Tycho is the youngest of four children. His parents have decided what each of them should be, and the eldest three have enthusiastically acquiesced. Tycho has rebelled and is disliked by his parents and bullied by his siblings. Then one day he finds a shiny object which allows him to travel in time. His first experiment with the past disturbs his present and he decides to stick to the future but discovers that he doesn't like the futures he sees--each one, he figures out, is a possible future and in each he is unpleasant and his siblings are his victims. He also discovers that changing the future seems to change the present also.
The tale ends when Tycho realises that what each future has in common is the presence of the gadget. He goes far back in time and makes sure it isn't planted in the first place. Then he winks out. The final picture is of him digging his garden with his sister--with whom he is now friends--and finding .... a lipstick case.
The tale is typical Sleator in that it forces the reader to think about mathematics and possible worlds, but it lacks the substance of his other books. The ending is too pat, to perfect. It fails even to be a "happy" ending in the conventional structure of those.