I was wrong about this being Chrysalis' (Alice and Hatter's daughter) book. This one is mostly a tale told by Danika to a group of fairy children. There's more to the story than meets the eye (something I figured out before it was said) but I'm not going to spoil it.
The story of Jericho, the man in the moon, and Calanthe, a flower fairy, isn't a happy one. They fall in love during the nights that Jericho is allowed to be away from the moon, only once a month. They are betrayed by one of Calanthe's friends and cursed because they cannot be killed (they are fated mates, and bound so that if one dies the other does as well). They soon learn that there are things worse than death, he is cursed to forever be the man in the moon, when his five hundred years of service was almost up (less than two weeks) and Calanthe's curse is something I'm not going to give away.
They don't get their HEA, but it's better than nothing.