For once an urban fantasy that actually is an urban fantasy, with not a poxy Doc Martened elf in sight, thank god: part very well researched police procedural, part Sorcerer's Apprentice - and this is a real apprenticeship, where results come from sheer slog and perseverance, not from waving a magic wand - part gruesome murder hunt, part otherworldly politicking, and part - and this is my favourite part by far - a carefully detailed map of Central London, written by one who knows and loves the place as it should be loved, all of it narrated by likeable Everyman Peter Grant, an adequate enough probationary constable who's just a bit too curious for his own good (while his colleagues are breaking up a riot in Trafalgar Square, he stops to check what's written on the lions' bums). Peter crosses the line between one world and another and, in the process, is saved from a future of data entry in the Case Progression Unit, one cold morning in Covent Garden, when he's left guarding a crime scene and a witness steps forward from St Paul's Church - a witness who has been dead for considerably longer than the body under investigation.
Rivers of London isn't perfect - there are a lot of open questions left hanging, and some of the plot resolutions don't entirely make sense - but it is bloody good, and one of the best things I've read in many years. Good enough that I went straight out and bought the sequel. In hardback.
Ignore Diana Gabaldon's stupid cover blurb, by the way. I'll do her the credit of believing she was misquoted.
Don't expect to learn too much about the actual lost rivers of London, though. That's a fascinating topic in itself, but one for another book entirely.