** spoiler alert **
This is one of those classic McKinley stories like Sunshine
, The Hero and the Crown
, or Deerskin
and etc. where the heroine is torn between two 'men' (or maybe male creatures would be a better term) and between 'real life' and 'other'. (It is a very common theme and conflict in her writing, as long-time fans will recognize.)
Since, reading between the lines, it is pretty clear that the relationship between Maddy and the Stone Fey is a physical one, it is odd that this book is usually shelved with the children's picture books. It's likely that younger children won't pick up on this, however. (Nothing more overt than reading between the lines in all of the Blue/Green/Red/Yellow/Whatever Fairy Books and the like.)
I may be wrong in this, but I have the impression that the story itself was written (and maybe published) then it was decided to do it as an illustrated book. That may be why it doesn't really work well (as far as storyline/theme) for young children?
It is a beautiful and mystical story with lovely atmospheric pictures and lovely for those readers looking to glom and/or acquire all of McKinley's backlist