I recently looked again at Journey to the Center, by Brian Stableford. This was one of the first sf books I ever purchased. It is the story of humans who live on an artificial planet, built by some unknown race millennium earlier. The planet has endless levels and passages honeycombing its interior. The center has never been reached, and in the circumstances of the plot, one explorer who makes his living searching for artifacts within the planet, is caught between several factions who desire to journey deep into its interior.
It is a book that holds up well. I did not realize until recently that Stableford had eventually written two sequels, now out of print. Seeking them out is on my agenda.
Stableford is or should be considered an sf master. He has published more than 50 novels since the seventies, and is also an editor, non-fiction writer, and translator. In these past five years, last I read, he has no contract with a publishing house yet continues to write novels on a publish-on-demand basis. I wish some publishing house would pick him up and give his latest books wider distribution, both because he is an excellent author and out of respect for a master author of sf and fantasy.