I recall bits from one of my dad's books that I read in the late 1980s.
It was about a boy who lost his thumb (or finger) when a door was shut on it. He joined a circus or carnival where he looked like a woman that was in an act in the carnival. The man that ran the circus/carnival asked to look at the boy's thumb and it had grown back.
I recall the man talking about a pair of trees: when he carved something in one of the trees, the other tree would have the same carving.
In the end, I recall the boy could control how he looked, like shape-shifting.
I’ve been searching for this book forever but cannot seem to find it. I asked my dad and he thought it might be a JRR Tolkien book, but I don’t think so.