The Beast: In the secret caverns of the moon lived a strange society of immortals. Each had been caught in the vortex of a weird time-machine which snatched its Earthly victims from their own eras and transported them to the lunar caverns of eternal life. Into this bizarre set-up stumbled Pendrake, an Earthman in the process of becoming superhuman...

A fix-up: source stories are "The Great Engine", the short novel "The Wonderful Man" (a.k.a. The Changeling, with the main character's name changed to Pendrake), and "The Beast" - all from Astounding from the early to mid Forties. A British edition was titled Moonbeast in 1978.

Here's a plot-teaser for The Changeling : Leslie Craig woke up one morning to find he was 20 years younger, and some sort of superman - able to regrow lost limbs. He is promptly abducted and brought to confront President Dayles. What Craig didn't know, and Dayles did, was that Craig was a toti-potent. By controlling Craig and keeping him ignorant of his true state, Dayles saw an opportunity for supreme power… But memories have a way of returning, and the truth will come out.