11/22/63 by Stephen King is a fascinating work of fiction about someone travelling back in time to try to stop the 1963 assassination of President John F. Kennedy.
Thirty-five-year-old Jake Epping goes through a portal that takes him to 1958 and takes the name George Amberson, living first in Maine (including the town of Derry, where It was set). In Maine, he attempted to prevent a young girl from being paralyzed by a straw bullet, and save the lives of a mother and three of her children, all murdered in their home. He then went to Florida and finally Texas (both Dallas / Fort Worth and the 1,200 person town of Jodie). While in Jodie, he met and fell in love with Sadie, a fellow teacher at the high school.
During his years living in the past, Amberson found it to be obdurate, it didn't want to be changed. As he would attempt to do things that would alter history, roadblocks would present themselves and have to be overcome. Also, changes made to the past would cause a butterfly effect, begetting other changes.
He went through the portal several times, with each new trip to the past resetting history, cancelling out any changes he caused during previous time travel. It was a compelling book and the ramifications of what might have happened had Kennedy lived are fascinating to consider, and written about by King in the postscript.
