Good to know that the entire royal family was cruel, selfish and not suitable for leading... 
 It follows the story of Queen Levana when she was a teenager. 
 * We get to know her sister, (Cinders mom). 
 * We find out how she got burned when she was little and why she glamours herself. 
 * How she met and fell in love with Winter's dad. 
 * How Winter and Cinder came to be and most of all 
 * We find out how truly horible all the royal family was and that maybe the way she is wasn't really her fault to begin with. 
 So you actually can skip this. It isn't important to the main story and you don't have to read it in order to continue on with the series. 
 But I still recommend that you do.