Ben Aaronovitch & Andrew Cartmel 
Rivers of London: Detective Stories #4.4 
Detective Stories #4

Undergoing his detective exams, PC Peter Grant has been looking back on some of his previous cases. The final one takes him back to one of his first – on that has nothing to do with his other profession: trainee wizard in the Met’s Special Assessment Unit. However, it is a time of remembrance past with his old partner, Lesley May – the same partner left mutilated by magic and who has now gone rogue, vanishing into London’s mystical underworld…