I think the standard phrases are 'unique voice', 'deadpan humour', 'fabulous', 'mythic', 'whimsical'. "Odd, endearing and disturbing" describes most of them. In my experience they can be infuriating to the point that sometimes you want to throw the book across the room (I did that once after a particularly deceitful reveal midway through 'Explorers of the New Age' ) but then rush across to retrieve and read on. They are very easy to read. The themes seem generally to be related to worlds of systems, routine, work, and threats to those worlds/societies from outside. There is usually a sense of impending doom as a crisis looms.The books are usually narrated by a protagonist who is trying to find his way in whatever society is being described, and struggling.
The Restraint of Beasts was the one most grounded in a world which is most recognizable as ours, give or take couple of gratuitous/accidental murders.
All Quiet on the Orient Express (my favourite) can be pinpointed in a modern setting and location but is starting to get a bit weird.
Three to See the King .Not recommended. Very weird. I've forgotten it apart from something about wanting to live in a house 'made entirely of tin'.
The Scheme for Full Employment. A couple of laugh-out loud moments. It's mainly about a system of routes for vans which are carrying spare parts for those vans, and cakes, between various depots. It has the given the team I work in now the phrase 'An early swerve' (eg I've worked late this week twice already so I think I'm going for an early swerve).
Explorers of the New Century. A sort of skewed Amundsen v Scott tale to be the first to reach the 'Agreed Furthest Point'
The Maintenance of Headway. Where Magnus Mills draws on his experience as a bus-driver (I think he still drives buses in London). The idea is to maintain the ideal gaps between the buses but the system that the inspectors are trying to impose doesn't always suit the drivers.
A Cruel Bird Came to the Nest and Looked In. Set in the made-up world of Greater Fallowfields. A model society except that nobody seems to be in charge - the Emperor is away somewhere - and nobody is equipped to do the job they have. Still, a pleasant enough place to live, except that the next society along seem to be building a railway through the woods directly towards Greater Fallowfields.
The Field of the Cloth of Gold. Halfway through. So far it's reminding me strongly of my experience of camping in Loughton before LEL 2013. That and it being a metaphor for imperialism and the Romans in Britain.
All strongly recommended from me (except Three to See the King) but don't blame me if you find them nonsense. A Cruel Bird, I think, is the most purely distilled Mills, so if you only read one more after The Restraint of Beats, then that should probably be the one. Oh, and if you think you an get tasters by reading his short stories - don't. They really are crap.