Divine Right by Lester del Rey

In a world where the privileged are granted immortality through cybernetics and secretive methods, Mark Saxon faces a career-ending failure. While appealing for more time to complete his research, he discovers evidence that the current World Custodian may have lived for centuries by transplanting his mind into new bodies.

As Mark races against time to expose this truth, he uncovers the depth of the custodian's power and the horrifying reality of mental immortality. With his life at stake, Mark must decide if he can trust those who promise him the ultimate gift: a shared mind and eternal fellowship.