Way Station by Clifford D. Simak

"Enoch Wallace

looked like any other man on Earth – except that:

--Time (about 100 years) had passed and he showed no sign of aging
--his house was invulnerable against destruction from any weapon known to man
--in his family cemetery a tombstone inscribed in an unknown language guarded the grave of an alien horror

For years no one had invaded the solitude of his lonely farm. But now the watchers in Earth were closing in on him, determined to solve the mystery of who or what it was that went under the name of Enoch Wallace—a man, magician, or nonhuman creature from an alien planet.

Way Station, Clifford D. Simak, (c) 1963