Book title and author sought!

[IzzyHandsIsMySpiritAnimal]

I read a book possibly 7-10 years ago. One of the old, classic type things and I'm desperately trying to recall what it was.
The only bit I can remember is that at one point there is a man living on a rock. He's either covered with hair or lichen (or both?) and he has managed to live by drinking or sucking water from the rock (or possibly also from lichen).
He gets rescued, he may have been chained up, and then is somehow redeemed and turns into a 'normal' man again.

I don't think it's Candide as I can't find those things in the text I'm looking at online but it may be from that time or earlier. I think it was a slim-ish book.

Sorry, don't have any more to go on, but if anyone has any ideas as to what I might have read, please let me know.

Could be The Homeward Bounders by Diana Wynne Jones.

I think you have posted this in the computer stuff area, not the book area, btw.

The Man of Adamant?

I thought geeky fitted the bill but yes, computers would make more sense in context! I'll see if I can get it moved.

I don't think that book suggestion fits sadly, this would be something written at least 200 years ago, possibly more.

The Prisoner of Chillon by Byron?

Unfortunately not. There are some similar elements but I think the book I read was older than this.

Sorry no. Not a poem, it was a book, I don't think it was particularly long. I think it's older than Byron's work.
Wish I could remember more as it'd be very helpful.

Not 200 years old but The Unnameable by Samuel Beckett is a possible fit.

There are some similarities but that one's not it either.

I think there was some travelling around involved, and the protagonist encountered this person. It has shades of Candide where he and Pangloss meet various people.

Not Robinson Crusoe? Or one of the episodes of Gulliver's Travels?

No, neither of those. Possibly more like Pilgrims Progress bit less obviously religious.