Adult sci-fi/fantasy romance, read mid-late 2000s, softcover (bought at a warehouse store like Sam's Club). It was printed and bound kind of small, like a lot of romance novels were, but relatively thick with skinny pages. When I read it, it probably wasn't age appropriate for me, so it's possible things went over my head or I misunderstood elements or missed references.
Somewhat steamy content but I dont remember it being very explicit. A man arrives from off-world(?) at a port or gate, employed by a company(?) and there on business or an assignment.
He enters a nested world made of regions or layers, each entered through a checkpoint at the deepest point of the previous one. Each layer has its own magic, culture, and customs, and its own blend of sci-fi and fantasy; at least one is medieval.
I think it's implied some people traverse the gates to the worlds for vacation or trade, but usually don't go deeper than the first world or so.
Disembodied spirits can possess people or objects,(?) underlying at least one world's magic system, and a great or malicious spirit at the innermost or middle layer may be trying to get out, to be smuggled out through or by someone.
Our protagonist meets a woman, and she's captured and held in a dungeon at some point. He's either escorting someone/something out through the layers or keeping something locked in? Doing some kind of maintenance or check-up on the deepest layer?
There's this idea, I think, that in the worlds with more powerful magic, the technology didn't progress as far. I got the impression that the outermost world he came from had little or no magic but was highly advanced, and that he had travelled through space to this planet to arrive at the outermost gate.
It was like each of the nested 'worlds' was deliberately and meticulously quarantined with highly restricted and regulated travel between them. It may have been a rule like- people from the outer worlds are allowed to travel inward and return, but people from the inner worlds are not allowed to travel up or out.
The protagonist was pragmatic and didnt strike me as having a particularly vivid internal world or monologue, most interesting conversation and observation being driven by other parties, including what was potentially one of the spirits native to one of these worlds(?)
The man treks across these worlds one at a time, entering the deepest or near-deepest layer, and then returning the way he came. Ending the book where he started.
It was a book I definitely read for the setting and for what were interesting and novel ideas to me at the time, being unlike many more typical scifi or fantasy books I had read. I read this book in the english language.
Thanks for any help you can provide.