An absolute delight to read. I had no idea what to expect of this book; I think I've read one of Aaronovitch's Doctor Who books, and I've seen the DW eps he's written, but obviously, this was something entirely different. And a very good sort of different it was.I loved, loved, loved the writing in this. The narrative voice of Peter Grant, the young London police constable, is wonderful - rich, realistic, funny, with a perfect balance of slang and bureaucratic jargon when necessary. He's also a thoroughly likeable protagonist, which helps a lot; reasonably well developed for the first book in a series, and his mixed African/British heritage only adds colour and spice, never seeming like a gimmick nor his only defining characteristic (which is, of course, as it should be and shouldn't need to be pointed out, but is unfortunately not always the case).
The supporting cast was excellent, too; I think I may have a slight crush on Inspector Nightingale, and I really hope we'll find out more of his mysterious background in future books.
The magic, if perhaps not staggeringly original, seems well thought out, and I enjoyed the reluctant cooperation between the ordinary part of the London Metropolitan police and the magical department (consisting of, well, Nightingale and Grant). The whole rivers of London part of the story was interesting and provided a nice secondary plot line.
I think the only part of the book that fell a bit short for me was the actual main plot, which started out excitingly enough but didn't really truly engage me later on. Still, everything else was fabulous, and it's one series I'm very definitely going to read more of - the best urban fantasy with a male protagonist I've read yet.