I have never read these. I remember being told about them a few years ago. These are a series of shared world stories where our earth is invaded by a parallel universe. The parallel universe has military technology at the level of muskets and I believe they have some kind of floating ships as well. The statement I remember is that it was 'natural' for them to develop that way.
The person also told me there was at least one story where the SWAT team beat them back. The parallel universe invading us had invaded other parallel universes also.