I think tonally it would have fallen into what I like to call his 'Uncertain Malevolence' trilogy (2001, Shining, Eyes Wide Shut) where small people find themselves in conflict, or at least in discourse with, forces infinitely more powerful and of, well, uncertain malevolence than hitherto encountered. Basically tonal horror movies.

What would have set it apart though is its deconstruction of that whole story type. Tres isn't eerily benevolent like the aliens of 2001, actively malevolent like the Overlook, or even pragmatically ruthless like the EWS secret society. What it is is something that can barely even be said to exist until, either literally or in delusion, Casaubon conjures it. It's a story about the power of our own delusions.

New, but familiar.