Whispers Underground is the third book in the Peter Grant series by Ben Aaronovitch. I devoured this book in little more than a day.
PC Peter Grant is called to the Underground while pursuing a lead about the wizard who’d tried to kill him during his previous adventure, and when he gets there he’s greeted by the terrible sight of a man stabbed to death. Though he doesn’t pick up any magical residue from the body, he does from what they assume to be the murder weapon. A shard of some kind of ceramic. With the help of his mentor Nightingale, his now fellow student PC Lesley May whose face is still ruined, the Murder Team and a strange bloke named Zach who is more than he lets on, Peter descends into the tunnels of the London underground and the sewers. All while being followed by a pesky FBI agent who has pegged him as suspicious.