I read this 10–20 years ago. It's a hard sci-fi book about a man — a physicist — who invents something that swaps spheres of matter with somewhere else. I think the swap was between universes, but it might have been between different times.
He increases the power of the device at some point and swaps a sphere containing himself; I think he ends up inside a tree. He also goes outside of the tree (on a different trip) — into an unfamiliar countryside. There was some kind of timer, I think, to swap him back.
Someone abducts him (I don't remember why, or if it's related to his invention), and he uses the invention to escape; he uses the device to swap part of his abductor's brain with something else, killing him.
I'm pretty sure there was a 1:1 mapping between locations here and the other place, so that when he went there, and he walked within that world, he would be returned to our time/universe to a different location (compared to where he started).
I read it in English, and it's a full length novel.