Firstly, let me say that this book isn't for everyone. The text is dense and can be confusing. I'm sure there are plenty of critiques that could be made about the depth of the characters, the pacing of the plot, the weirdness, the deus ex machinas, the sea. I'm not going to make those critiques. Seek those details elsewhere.
This was a book that was a joy to read, not so much for the content as for the manner it was written. When Perdido Street Station came out, I reveled in the way that Mieville examined and explored the amazingly creative world he created without ever describing anything. We learned sentence after sentence, page after page, by little more than context clues. The story plowed through the world as it would, and we figured it out or we gave up. It was a wild pleasure, a native's-eye-view of the mad city that clearly existed whole cloth in the author's imagination. In Kraken, for really the first time since Perdido Street, I am once more caught up in the chaotic power of Mieville's ability to write without apparent regard for his audience. Instead, Mieville writes like a lover possessed and possessive. He mangles the language in beatiful ways. Fragmentary sentences, verbs very optional, are stitched together with London slang. He never misses a chance to use a five dollar word when the sound of it is more musical than a fifty cent word.
Kraken is set in modern day London, and a giant squid is stolen. In a way that feels very familiar to anyone who has read Gaiman, an everyman Londoner is brought suddenly into a world of hidden magic lurking in the secret places beneath the surface of London. I enjoyed the story in its own right. The magic of his London is powerful, striking, and (as the protagonist points out at one point) almost disappointingly obvious. The weirdness is like much of Mieville's work: neat. The creatures living in London reminded me a bit of The Weaver from Perdido. The characters surprised me in several instances, and made me smile often.
But that's not why I loved this book. It isn't for everyone. If you like your plots water-tight, your pacing metronomic, and your prose accessible... maybe skip this one. If you're in the mood for a weird modern fantasy about the end of the world written by a man who knows the worth of a good word... pick it up right away.
 Geeky moment of the review: in many ways this book reminded me of Tycho's writing in Precipice. A love of the language melded with a passion for the apocalyptic.