The Asgard Trilogy by Brian Stableford.
It consists three books with various titles: Asgard's Secret (1982), Asgard's Conquerors (1990), and Asgard's Center (1990); or it may be Journey to the Center, Invaders from the Centre, The Centre Can Not Hold.
I was excited last year to learn this was a trilogy, as I very much enjoyed the first book when DAW published it in 1982 as Journey to the Center:
Here is the Amazon description of first book:
They call it Asgard, the Home of the Gods. Beneath its artificial shell are at least three vast cave-systems, each one the size of an Earth-like world; and beneath those, possibly many more. No one knows how many layers there might be, and no one knows what secrets might be buried down at the "center"--if there is a center.
At some time in the distant past, Asgard had suffered a terrible catastrophe. Now its outer layers are cold, its builders presumed dead. Explorers and exploiters from a hundred different worlds and races are scavenging among the ruins--but deep down, there might still be light, warmth, life...and perils unknown.
Mike Rousseau was one of the first humans to come to Asgard, decades earlier. Now he's challenged by both friends and foes--and it's hard to tell them apart on this icebound planet. He might also be the one man who can solve the biggest puzzle in the galaxy...if only he can stay alive! The first novel in a thrilling trilogy of science fiction adventure and discovery