The privileged aspect of the group of friends is intriguing. They are off on a vacation, Cassie is showing her bravery and perhaps foolhardiness scaring her friends while climbing, and then inexplicably, things go very, very wrong on their way back home. It seems like a good idea to be able to have enough money to charter a private plane, but maybe not! To paraphrase Fitzgerald, the rich are very different-- they have more enemies.
There definitely need to be more suspense books for teens, and this joins the ranks with Evans' Michael Vey series, McEwen's Camp Valor, McNab's Traitor series, and Coben's Micky Bolitar novels.