Skeleton Song
Seanan McGuire
Skeleton Song is the origin story for Christopher of Seanan McGuire's Wayward Children series. We met Christopher in the first book, Every Heart a Doorway, and he and his world Mariposa played an important role in Beneath the Sugar Sky. Christopher carries a bone that he plays like a flute (although it emits no sound that any ordinary person can hear). And he proclaims his love for the princess of Mariposa, who is a skeleton. Everyone in Mariposa is a skeleton.
Skeleton Song tells of Christopher's coming to Mariposa, of his love for the princess, and of his departure. It is rather better than average, I think, as Wayward Children books go -- hence my four-star rating.
Although this story precedes Every Heart a Doorway chronologically, I would not recommend reading it before the main novellas of the series. You should at least read Every Heart a Doorway, and probably also Beneath the Sugar Sky to understand the premise of the series.