I am already a huge fan of Marie Lu. I love her writing style--always so beautiful, yet crisp and clean in its execution--and her story ideas. However, this is the first I've read of hers that is not straight sci-fi/fantasy!
The Kingdom of Back follows Nannerl Mozart and her brother, Wolfgang as they develop their musical talents. Although most know at least a part of Wolfgang's story, few have heard of Nannerl despite her impressiv talent, mostly due to the fact that she is female. This weighs on young Nannerl and she finds escape and hope in a different world in this portal fantasy lush with music and imagination, both described in incredibly beautiful detail.
Nannerl is a sympathetic protagonist who stole my heart and had my compassion early on. She is very easy to root for with her many admirable AND flawed characteristics. My only complaint was that much of the story follows Nannerl as an 8-year old, which made it difficult for me to identify with her.