One of his top five books about achieving immortality, as shared at Tor.com:
Riverworld by Philip Jose FarmerRead about the other entries on the list.
In Philip Jose Farmer’s Riverworld we are resurrected to eternal life in new bodies on a world covered with a giant river valley. We awake on the banks of this river in perfect 25-year-old bodies, which can regenerate from just about any injury, and remain at that age forever. No severed limbs, no wheelchairs, sight restored if lost, no tooth decay. Provision is even made for those who died young. They are resurrected at the age they died, grow to 25-year-olds and so remain. This is a direct translation of religion into an sfnal setting, and its source is hinted at all through. The resurrected get their sustenance from “Grails,” while one wonders why all the men are reborn without foreskins. The heavenly host, the alien Wathans, created the place for “moral contemplation” where humans can lose their barbarity in preparation for moving on. But still, that contemplation involves numerous adventures with historical figures—the explorer Sir Richard Burton is a main character—and the books are an enjoyable ride.