Un Mundo Fuera Del Tiempo by Larry Niven

In a distant future where humanity has achieved immortality through the transfer of consciousness into new bodies, a man from the 20th century is revived and tasked with a mission to save Earth from a dying sun. As he navigates a universe transformed by time and technology, he encounters a society vastly different from his own, grappling with the ethical implications of immortality and the consequences of unchecked technological advancement. His journey through space and time challenges his understanding of identity, purpose, and the very nature of existence.