[Spoiler warning - usually don't bother but I'll be revealing some major, uh, revelations.]
China Miéville's Perdido Street Station (2000) is a messy, moderately imaginative novel that's acquired a reputation out of proportion to its real achievements. It's a self-conscious mix of fantasy and SF elements - the main threats are quasi-Lovecraftean entities who eat dreams and a possibly renegade AI just as the two main characters (up to a point, noted below) are an artist and a scientist. Basic story is that Isaac the scientist is approached by Yag, a bird-man who was convicted of an unnamed crime and had his wings removed, to find a way for Yag to fly again. From this everything else starts to build even though the main conflict is completely accidental to this initial purpose.
And that twist, which occurs nearly at the half-way point, is one of the problems. The entire first 200 pages are an elaborate stage setting that really shouldn't have been but a quarter of that length. It's no revelation that editors today don't want to do the real work of editing but don't at least some of them remember Max Perkins? The other issue is that the second half is mostly a completely different kind of book, more an adventure story about stopping unstoppable monsters. I'm not completely joking by saying I think you could start reading at page 150 and still get the full impact.
Like so many F&SF writers before him Miéville has become too fond of his setting for the book's good. When he wanders off on a brief summary of some neighborhood's history it's almost like he was writing a Rough Guide and though many people say the book reminded them of Dickens (some of those are even excerpted for blurbs) that's only superficial. Dickens was a master at secondary characters but Miéville rarely creates anybody who doesn't play some part in the overall plot - thus the gangster who appears completely unrelated is an important element later, we're treated to a lengthy (and completely irrelevant) description of one of the mayor's advisor's secret life while hunting the monsters, a stray journalist becomes a key character (while the artist fades away). Even a trio of mercenaries that are basically just plausibility support get a recount of how they met. This is one author who really needs to be studying Hemingway. Still, having few true secondary characters for a book this long creates an oddly compact feel, especially when he should be going for expansive. Lots of writers were able to do this even better at shorter lengths - Leiber, Vance, Harrison, Moorcock, etc - but sometimes writers don't learn the real lessons.
I would also recommend not expecting Perdido Street Station to be as unusual as you may have heard. Miéville is often lumped in with the New Weird movement/tendency (though apparently everybody in that lump claims they're actually not part of it) but there's not a lot genuinely weird about this. OK, the artist character does have a person's body and an entire insect for an head while a gigantic sentient spider fills a trickster god role but really the whole book is fairly conventional fantasy, almost steampunkish. The city does resemble a 19th century London, the goverment is a straight-forward police state, professors teach at a recognizable university, people ride trains and carriages, etc. Magic is treated as a kind of science. And so not a weird effect at all except in descriptions of the non-human ("xenian") races.
The real kicker, though, is the closing sections of the book where another bird-person appears to reveal Yag's crime - this second bird-person appearance is so implausible that Miéville has his character comment on it. Why not have Yag reveal this? As it turns out Yag is a rapist so while Isaac now has a way to allow him to fly he's in the dilemma of whether to not do it because of Yag's crime or do it because Yag was a client and a good friend, stood by Isaac during extremely tough times, saved the city from certain death as well as separately saving Isaac's girlfriend. This being a 2000 novel we already know what the decision will be but the whole thing is so oddly contrived that I can't but wonder what Miéville was thinking. There's nothing else in the novel that's really a moral decision (questions of racism are brought up in the Isaac-artist relationship but Miéville apparently does that just to denounce the small-minded, who incidentally we don't meet). Did Miéville honestly think he was doing any hard work here? Did he want a "everything you know was wrong" moment? Did he just feel there needed a better closure to the whole book?