I have posted this query on Reddit and on Stack Exchange previously.
This would have been after 1996 – 2005 ish, and I’m pretty sure it was a newly published book.
It starts with a woman who has hallucinations, and runs into an alleyway where she finds a body that looks just like hers that has been murdered. Later on, she finds a beautiful man who says he’s from another parallel universe and seems to think he’s connected with Arthurian legends, i.e. he talks about Merlin, wizards, and so on.
It starts off looking very magical and predictable, with the woman and the handsome man trying to hunt down the murderer. Then it goes somewhere else entirely.
There were seven murders in total at various points around London — they thought that it might be Jack the Ripper based, but it turns out the murders occurred at ritually significant distances around the city, and that Jack the Ripper was actually the last time they tried this, to open a portal between the worlds.
They think that the slimy enemies from another dimension are responsible, and the handsome man calls to Merlyn, his boss, to help out and ensure the slimy enemies don’t get a foothold.
They succeed, but then the woman finds that the slimy enemies are not responsible for the murders. The handsome stranger turns out to have murdered her clone himself as part of the ritual that would allow his reality to enter hers — she is actually the clone, and the original was murdered. Her confused memories and hallucinations are because she ran off before the stranger could finish creating a convincing replacement and so she still retains some memories of the original.
The handsome stranger is actually a useful idiot that is handsome and dumb precisely because he’s a facade that covers what the traders actually look like (they’re distinctly non-human and not nearly as affable). He’s a construct.
And there’s the Handsome Stranger’s manager… who appears as a kindly wizard based on Merlin? Again, he turns out to have been orchestrating the entire thing, and the whole “help us save your reality against evil slimy things” turns out to have been a dispute over trading rights to Earth between two equally slimy organizations.
The book ends with humanity establishing trade and goods with the unbound reality and becoming less and less human — strange buildings, humans with gecko like arms and legs living on walls, etc. Very creepy, and very hard to forget.
