The Chalk Man by C.J. Tudor

In 1986, Eddie and his friends are just kids on the verge of adolescence. They spend their days biking around their sleepy English village and looking for any taste of excitement they can get. The chalk men are their secret code: little chalk stick figures they leave for one another as messages only they can understand. But then a mysterious chalk man leads them right to a dismembered body, and nothing is ever the same.

In 2016, Eddie is fully grown, and thinks he's put his past behind him. But then he gets a letter in the mail, containing a single chalk stick figure. When it turns out that his friends got the same message, they think it could be a prank . . . until one of them turns up dead.

That's when Eddie realizes that saving himself means finally figuring out what really happened all those years ago.