An excellent book by Clifford D. Simak: Time and Again. Serialized in the first 3 issues of Galaxy magazine as "First He Died." I'd say this is one of Simak's 2 or 3 best books, and easily makes my personal top 100 list.

This is the story of Asher Sutton, the only man to return from the 61 Cygni star system. He comes back 20 years late, in a leaky spaceship (and no spacesuit.) He is expected by 2 rival groups of time-travelers - for he will write a book that will change mankind's future, and each of these groups want it written their way or not all... and one group will kill him if they must. But Asher Sutton is not easy to kill: he did not come back alone - he came with his Destiny.