I didn't nominate Perdido Street Station, but I'm starting the discussion at the request of the Person Who Did. So ... here we go.
Let's start with the obvious - Perdido Street Station is a big book. There's a hell of a lot to chew on in here, from politics to the workings of fantasy, and it's very difficult to know where to start. Indeed, that is one of the glorious strengths of the book: at times dizzying in its scope and creativity, this is less a novel and more a sprawling map of words. Geography and topography, society and culture, politics and science ... New Crobuzon has a life of its own in all of these areas, and we grab glimpses of all these and more as a new visitor to a foreign clime. Perdido Street Station makes things feel real and tactile; its language may be dense and at times over-wrought, but it is perfectly in-keeping with a novel which revels in its baroque complexity.
Mieville's almost forbiddingly dense, but I'd argue that he's also eminently accessible - perhaps unlike the obvious comparison, Mervyn Peake's Gormenghast novels. It seems to me that the first 200 pages of the book - almost a quarter of its length - is taken up solely by the imagining of New Crobuzon. Some of the book's best writing is here - the novel's setting is in many ways the principal character and it and its inhabitants are beautifully and enticingly drawn. I've read both of Mieville's other Bas-Lag novels, and never felt quite the same sense of breath-taking wonder.
But isn't this also the book's principal weakness? 200 pages and hardly a whiff of plot? That's just self-indulgent, surely? The prologue is a case in point - it's doing its level best to be mysterious and creative and act as a moving introduction to the city, but in fact it comes off as drawn-out, opaque and unnecessary. OK, New Crobuzon's great, but what I really want to know is what happens in it and why. I want a story, not a guidebook. It's well known that Mieville based New Crobuzon on London - he can write a fantastical love letter to the city as much as he wants, but if it don't got no story I don't see why I should read it.
Similarly, the characterisation in the book is sparse: Isaac is thoroughly likeable, as a hero should be; Lin is a damsel-in-distress, dealt with atrociously seemingly for the sake of damaging a character we care about and making a simplistic villain out of Mr Motley; Lemuel Pigeon is the loveable rogue, Yagharek the enigmatic visitor. For all its invention, Perdido Street Station is also a rather simplistic novel - its characters are archetypes and, once it gets going, its plot is pretty straightforward. If you remove the asides - the entertaining visit to Hell by the Mayor and his compatriots, for example - you're left with a linear plot of the most average kind.
Still, there are surprises, some of them more baffling than others. The ending, for example, comes out of left-field for many. Having been a secondary character for much of the novel, Yagharek takes centre-stage in what can feel like a fairly rushed and even pointless denouement. Structurally, Perdido Street Station isn't really very strong, is it? It steadies itself on the strength of its creations, but in truth it's wobbling all over the place. The book isn't episodic (or is it?) or picaresque (or is it?), but there is a feeling that Mieville is more interested in his setpieces and his clever (and often strung-out) writing than he is in crafting a novel.
Crucially, though, Perdido Street Station is in the end about something more than its world. It is perhaps primarily a book about the the destructiveness of the will to power. Many of the characters of the novel are interested in imposing order onto chaos. They attempt to find patterns, force logical progressions, press societies into strict and unyielding systems. Ultimately, this is shown to be not quite an exercise in futility (indeed, despite the bustling confusion of New Crobuzon, the totalitarian government and its secret police seem to rule quite effectively), but rather one of denial. Thus, it is crisis and the impossibility of resolving impossibilities that save the day. The book embraces the complex interplay between conscious and unconscious, the rational and the irrational, rather than endorsing one over the other. Order must necessarily be restrictive, but there are varying degrees of order and varying degrees of restrictiveness. The book spends much of its time attempting to decide where on this gradient societies should place themselves to best realize their potential. There is perhaps no 'best' place - like much else in the book, the answer is one of personal choice, not empirical wisdom: "I ask you to observe our justice, not to impute your own."
Fantasy very often develops an inward-looking myopia, becoming increasingly interested in its own reality rather than the one in which it is written and will be read. Perdido Street Station can be accused of losing sight of structure and brevity, and even characterisation, as a result of its obsession with creation, but it is at least engaged with debates we can recognise as our own. Do we side with the 'justice' of Yagharek's society? If not, why not? Is it better or worse than the oppression on show in New Crobuzon? Is there any way to compare the two poles? If not, how do we make any political decision? And, besides, how can any over-arching system operate correctly and/or fairly when the individuals who live under it are constantly causing such chaos? This is an explicitly political book.
It's also the spearhead of New Weird, that amorphous branch of fantasy that gets certain people terribly excited but seems in fact to be a simple trick of emphasis, a deft shift of backdrop. As noted, plot-wise and character-wise Perdido Street Station is not ground-breaking - a gang of archetypes run about a bit, seeking to apply a McGuffin to some nasty monsters, in order to save the world against the odds. But they travel in a city that's part steampunk, part folklore, with bits of absolutely everything else thrown in, too. Isn't New Weird is just everything that's ever been written in speculative fiction thrown together in a splash of enjoyably bewildering colour to mask the fact that fantasy relies on certain character types and certain story types? Like the book in question, it seeks to distract us whilst holding our hand between the usual A and the much visited B.
In other ways, Perdido Street Station is nevertheless as unpredictable as its city. Sometimes, quite frankly, it can be rubbish. At others, it can be brilliantly entertaining. At still others, it can actually be thoughtful and incisive. It is an acquired taste - do we give it points for being so uncompromising, or is it arrogance to give no quarter to the reader? Is it a great book? I'm not sure.
But it's certainly big, and there's a lot to say about it. So get to it.