Ben Aaronovitch's bestselling Rivers of London urban fantasy series

When two young girls go missing in rural Herefordshire, police constable and wizard-in-training Peter Grant is sent out of London to check that nothing supernatural is involved.

It's purely routine—Nightingale, Peter's superior, thinks he'll be done in less than a day. But Peter's never been one to walk away from someone in trouble, so when nothing overtly magical turns up he volunteers his services to the local police, who need all the help they can get. 

But because the universe likes a joke as much as the next sadistic megalomaniac, Peter soon comes to realize that dark secrets underlie the picturesque fields and villages of the countryside and there might just be work for Britain's most junior wizard after all.

Soon Peter's in a vicious race against time, in a world where the boundaries between reality and fairy have never been less clear . . .