This Immortal by Roger Zelazny won the Hugo award in 1966. It shared the award with Dune. That should tell you something. It is also Zelazny’s first novel. In it you will find many elements of Zelazny stories that make him one of my favorite authors.
The setting starts in Greece sometime in the future. There has been a smallish nuclear war sometime in the past and much of the mainland is still radioactive (hot) so most people live on islands or previously less populated areas.
It is told in the first person by Conrad. We learn that Conrad was born at some (indeterminate) time in the past and was born on Christmas day. He also seems to not be aging. This may or may not be due to a radiation mutation. He is currently the head of the Earth Office of cultural sites and antiquities. He is being tasked with guiding a Vegan (not vegetarian, as in from the star system Vega) to various sites.
After the war, the Vegans stepped in and (at least to some extent) provided assistance to a struggling Earth and its colonies on Mars and Titan.
The story is suffused with references from Greek mythology. As the journey progresses through Egypt and back to Greece we gain insight into what has been going on, Conrad’s role and the politics that are happening.
This isn’t a long book by modern standards–a couple of hundred pages. Like many of Zelazny’s works, however, it packs in a tremendous amount. Zelazny paints a picture here that many would take hundreds of more pages to do currently. The writing is fun and poetic and well wonderful as the best Zelazny is. Sadly, it is currently out of print, but this book is well worth finding if you can.