Unique.
The protagonist, Frank, is most definitely a psychopath and yet one that I could almost understand their weird justifications - and that's credit to Banks, otherwise this could easily have been a story of a complete whack job.
The book is described (possibly by the publishers) as a gothic horror - it's certainly dark, but not sure it fits into horror.
It's definitely a mind fudge, and one that had me wanting to read more to try to understand if Frank understood what he was or not.
Definitely interesting, probably would have had a big impact if I'd read this in my teens (and not a 40 year old), still enjoyable in a challenging kind of way.