The Hunter (Book 1: The Guardian Series):

Something inhuman is haunting King's Cross, killing people in gruesome ways, Clare's friends are being chased by unknown Watchers, and, more annoyingly, an angel has fallen through Clare's roof. The danger that is circling Clare is unlike anything she has ever faced before, and she wonders if she will be enough to stop to apocalyptic vision of the future she keeps seeing. This is the first book of The Guardian Series.