Ok, so when I read this book it was called Rivers of London....
It's a decent book, focussing on a senseless murder, from which the narrator, Probationary Constable Peter Grant, is drawn into a world of magic, ghosts, revenant spirits, vampires and river gods. It's a perfectly enjoyable book, decently paced and plotted, populated by engaging characters - some of whom, particularly the somewhat ambiguous Molly and to a lesser extent Nightingale are intriguing enough to make the second book a definite to-read.
In fairness, it's not the best written book in the world but thoroughly enjoyable and readable