I really like the cross-pollination of crime and fantasy that Ben Aaronovitch achieves in his Peter Grant series, kicking off with Rivers of London. Peter is a young probationary constable with London's Metropolitan Police when a man is found dead in Covent Garden with his head lopped off. Peter is part of a police cordon of the area when he finds himself chatting to an eye-witness who also happens to be a ghost.
About this time, Peter is due to be allocated to a branch of the Met. Keen to do anything to prove his worth and avoid relegation to the Case Progression Unit, a job involving more paperwork than action, he decides to revisit the crime scene on the off chance his ghost will turn up again. Instead he meets Inspector Thomas Nightingale and his life is changed forever.
Nightingale takes him on as an 'apprentice', and if that sounds a bit Harry Potter, then that is because Nightingale is a wizard, in charge of his own branch of the Met which deals with ghosts, vampires, demons and the like. Relieved to avoid the Case Progression Unit, Peter moves into the Folly, a rambling old building that has plenty of secrets, including a strangely otherworldly housekeeper called Molly, a library full of musty magical tomes and a laboratory.
As well as wearing impeccable suits, carrying a silver-topped cane and driving a Mark 2 Jag, Nightingale can do magic. In the lab, Peter practices the spells that will help him compete with the darker forces, and destroy more than one cellphone. And he will need to get on top of that magic as soon as he can because the darker forces in this case soon take several more lives. The typical MO involves turning a victim into a raving Mr Punch (of Punch and Judy fame), their facial bones cracking into Mr Punch's hawkish profile, before committing murder.
Peter also has to step in to arbitrate when there's a patch war between the deities of the river aligned with Father and Mother Thames. It's lucky he's had all that training in conflict resolution, because the river gods become useful allies as he hunts down the spirit forces behind Mr Punch.
There's a lot going on, and you need your wits about you to keep up, or else you can just go with the flow - there's a lot of water in this story - and enjoy the witty dialogue and theatrics that form a key part of the plot. While this is very much a fantasy novel, Aaronovitch describes a very recognisable London and his police references ring true. This makes the novel quite accessible for readers who don't usually dabble in this genre.