Science Fiction FTW
A man uses science to do something that humans can't yet do with science. Doesn't get more science fiction than that. (Although we have to admit, we're getting kind of close to that power of invisibility.)
So this same guy from the science fiction world uses his impossible discovery for evil. That's some straight up horror right there. A lot of this book edges pretty close to comedy (in terms of "Tone"). But for the most part, we're downright frightened of the idea of an Invisible Man who wants to use assassination in order to take over the world.
On top of the whole evil monster taking over the world thing, there are a lot of other horror-like things going on in this book: uncertainty, ghosts, and madness. Put those together and you've got a major horror mash-up.