Well......... To be absolutely honest.......... I don't know!
I suppose you could call it fantasy, but probably this is just the easy solution. It clearly is a fantastic story, but that doesn't say much for the book, does it?
As I said in another thread, it didn't work as a horror story for me. I then googled it and found that someone has even described it as a love story! Or as a satire of academic criticism (I actually loved that and I really admired the effort required to construct all these titles of articles in the footnotes).
But my favourite description was definitely that one: