Rivers of London by Ben Aaronovitch

The provided excerpts from "Rivers of London" by Ben Aaronovitch detail the experiences of Probationary Constable Peter Grant as he becomes involved in a supernatural murder investigation led by Detective Chief Inspector Thomas Nightingale, the sole member of a secret Metropolitan Police branch dealing with magic. Initially called to a headless body discovered by a drunk witness, Grant encounters a ghost named Nicholas who offers spectral testimony, leading to his apprenticeship under Nightingale, where he learns to harness magic. The narrative covers their investigation into a series of "Punch and Judy" themed attacks perpetrated by a revenant spirit (Henry Pyke) and a vampire (Brandon Coopertown), while also introducing a complex hierarchy of London's river spirits, including Mama Thames and her daughters like Beverley Brook, who play a role in both magical agreements and police work. The story culminates with Grant being recruited as Nightingale's apprentice after participating in a dangerous ritual to defeat the revenant and witnessing magical fallout that nearly kills his colleague, Constable Lesley May.