Keeper of the Lost Cities (books)

The Keeper of the Lost Cities series is a middle-grade/young adult fantasy book series by American author Shannon Messenger. The first book was published in 2012 to great acclaim.

Synopsis
The series follows protagonist Sophie Foster as she discovers her true identity as an elf. She is taken to the Lost Cities, also known as Shangri-La, where she finds that she is part of a larger plot which some would even kill for.