This prequel to MY BEAUTIFUL ENEMY is a fantastic addition, because Ying-ying and Leighton's backstories are fascinating and heart-breaking in their own right. In my opinion, you should definitely read ENEMY first, but this is a wonderful, wonderful addition. I finished it in one day, because I couldn’t bear to put it down, and it makes me want to read MY BEAUTIFUL ENEMY all over again.
MY BEAUTIFUL ENEMY is one of my favorite books...ever, and I don't say that lightly. I love these characters passionately, and I love the setting (19th c London and Chinese Turkestan!), and I love their Second Chances romance that's shot through with tragedy and grief and regret. Absolutely marvelous from start to finish.