Perdido Street Station is, at heart, a thriller and horror story. It is science fiction and urban fantasy in its world building--this is not a thriller that takes place in a conventional Earth setting. Instead, New Crobuzon is a large city on some other planet--nothing is mentioned about its relation to Earth at all, but references to other nearby regions lead to the conclusion that it does not take place on Earth. And there's no space travel of any kind, no notion of life anywhere on any other planet. New Crobuzon is populated with all sorts of intelligent species, some of which you might guess were manufactured somewhere, and others you specifically know are manufactured. Plus humans. You gotta have humans, if only as a reference race, something the reader can latch onto and say, "OK, I know who these are, and now I know how they relate to the other races." It's really hard to write a book about alien races without introducing humans as reference, as foil.
Most of the "technology" is really fantasy. Kinda steampunk, but much richer than that. This is mainly fantasy that *looks* like science fiction. No elves here, but there are khepri--humans with insect heads. Fantasy.
But as I said, it's a thriller and horror story. It starts out a little slow, as there is a lot of set-up required for this story in Act 1. Yes, it uses three-act structure. Boo! Hiss! And because of that, the first 211 pages (of the paperback) are slow and difficult to get through. Until then, there's no big action sequence. Is it worth the wait? I decided it was, but I really think it would have been better with some more explicit action. That's not to say some interesting things don't happen in those 211 pages. They do, and they're compelling, and I still recommend you get through it.
Act 2 (3-act structure, boo! hiss!) is a monster movie. We need to fight the big bad monsters that threaten the city. But they don't just threaten the way a few bad mobsters would. There are mobsters and all kinds of unsavory types in New Crobuzon, and while they're all destructive and make life dreadful, miserable, and dangerous, by and large, despite all the dysfunctionality and filth and morbidity, the city moves along, and you could even say thrives, in it's own way, the way mold thrives in the walls of a building. No, these monsters threaten the very existence of the city--not in a cartoonish, there's a matter/anti-matter explosion threatening the city sort of way--but in an insidious, permeating, draining sort of way (avoiding spoilers here), which is what makes it a horror story in addition to a thriller.
Perdido Street Station is dense. It can be hard to read at times, especially with an SAT vocabulary word appearing every 5 pages or so. Mr. Mieville seems to have forgotten to always use a less obscure word when it will convey the same meaning. Because you can only convey meaning if your reader understands what the heck you're saying. And to force the reader to look up a word every 5 pages severely breaks up the storytelling. Maybe if one out of every 30 pages. That would probably be fine. But, seriously, I started counting once I got to page 150 or so: 93. That's how bad it was.
So it's a thriller and horror story, with a science fiction and urban fantasy background. It's a dense read (I had to re-read some pages and sections), it has a lot of obscure vocabulary words, and it's long (probably about 200K words). This isn't a book you read in a weekend.
But man is it fun! As thrillers and horror, it's one of the best. As sci-fi and urban fantasy, it's a great setting. And it's a perfect blending of the four genres. I found it in the library in "Teen Fantasy." Well, you can put it in thriller, horror, sci-fi or urban fantasy, but it's not "Teen." I suppose someone thought all urban fantasy was "teen." But this is not.
I recommend this book. I give it five stars despite its obvious flaws, because, in execution, this book is really, really good.