Humans live underground after devastation on the Earth's service. There are three decending layers - each a separate realm but ruled by the Sun Dwellers. What starts as a glace between two people -- Tristan (son of the Ruler of the Sun Realm who is a despot) and Adele (prisoner in the Moon Realm after her parents were branded treasonus), turns into much much more. 
 Full of battles and strong characters with bigger social implications, the story is very strong told alternately from tristan and Adele's points-of-view. Action packed and characters you care about.