You Owe Me a Murder by Eileen Cook 

17-year-old Kim never expected to plot a murder. But that was before her boyfriend dumped her for another girl. Now, Kim’s stuck on a class trip to London with him and his new soulmate and she can’t help wishing he was a little bit dead, even if she’d never really do that. But when Kim meets Nicki, a stranger on the plane who’s more than willing to listen to Kim’s woes, things start to look up. Nicki’s got a great sense of humour, and when she jokes about swapping murders, Kim… Read more.