Beyond The Fall Of Night by Gregory Benford

In a far future where humanity is divided between an insulated, static city and freer pastoral peoples, a curious individual breaks the boundaries of his world and embarks on a voyage that unearths buried histories and ancient technologies. His discoveries trigger contact with other intelligences and force a confrontation with long-dormant threats, prompting hard choices about memory, mortality, and the direction of civilization as competing visions of human destiny collide.