The Midnight Mayor by Kate Griffin

Second Hand 

It's said that if the ravens ever leave the Tower of London, then the Tower will crumble and the kingdom will fall. As it happens, that's not so far from the truth . . .

One by one, the magical wards that guard the city are failing: the London Wall defiled with cryptic graffiti, the ravens found dead at the Tower, the London Stone destroyed. This is not good news. This array of supernatural defences - a mix of international tourist attractions and forgotten urban legends - formed a formidable magical shield. Protection for the City of London against . . . well, that's the question, isn't it? What could be so dangerous as to threaten an entire city?

Against his better judgement, resurrected sorcerer Matthew Swift is about to find out. And if he's lucky, he might just live long enough to do something about it . . .