"The 13th Immortal" by Robert Silverberg

The 13th Immortal is set several hundred years after a century of war has caused the remainder of the world to retreat to technological stasis. Twelve immortal men have parceled the world into twelve domains, and they in their various ways have enforced an agrarian lifestyle on everyone. The thirteenth domain is Antarctica, newly green and secure behind an impenetrable field.

Dale Kesley is a farmer in Iowa. But he has surprisingly little memory of his past life. One day a man turns up, looking for someone -- for Dale. This man is from Antarctica, he claims. And so too, says the man, is Dale. And it's time for him to go home. After some internal agonizing, Dale decides to follow this man -- mostly because of a nagging feeling that he doesn't really fit in Iowa.

But their travels do not go smoothly, In South America the two are separated, and Dale is captured by the agents of the Immortal in charge there. Rather implausibly, this man takes a shine to Dale and instead of having him executed after an escape attempt he recruits him -- as an assassin! Dale's new job is to go back to North America and kill the Immortal up there!

It is clear that relationships among the Immortals are fraying. And during Dale's travels he learns even more about his world, as he ends up encountering a town full of despised mutants, and a town run completely by automation. Inevitably his peregrinations lead him to Antarctica, and a confrontation with the mysterious 13th Immortal -- as well as a realization of his own history and destiny.

This is really pretty minor stuff. Silverberg of that era was a competent craftsman, and often willing to at least make a stab at handling interesting issues -- but still often a producer of yard goods. This book is yard goods, and indeed a bit below the average Silverberg 1950s standard, perhaps not a surprise coming so early in his career. (Strange At Extaban)