Book about a guy in prison, on a jungle planet, with black light-absorbing walls?

Trying to locate an old book I read before.

I vaguely remember reading a book years ago about a guy sent to a prison planet.
It was no ordinary planet. The walls of the prison were black, absorbed all light, and were known to make people "crazy". While there, he was able to meditate and "connect" with something in the void walls of the prison, while others could not. The planet was the worst kind of planet, some type of jungle, and something could either kill you or poison you. But for this main character, it was nothing. He had seen worse.

But the meat of the book was him figuring out that the prison wasn’t built, it was found, and the walls, as mentioned, were not what they thought. I remember, and I know the old saying "don’t judge a book by its cover", but if an artist could connect with me visually, I would read it as a kid. The cover, from memory, had a jungle planet and a square prison with lights, with a guy cross-legged, maybe in a "space suit" or something (no helmet), and it was black/silver with raised lettering perhaps! I think he helped a lady escape as well, but I don’t remember too many details about it.

The book itself was WELL above my level of reading comprehension at the time, I was in middle school perhaps, but enjoyed every minute of it, because I remember it well into my 40’s.