As others have said, its a retelling of the Hans Christian Anderson story The Nightingale set at the court of Heian period Japan. It draws pretty directly from The Tale of Genji, with many of the same devices; a group of ladies in waiting who comment on and gossip about the actions of the main characters, a girl of moderate birth who rises above her class but feels insecure and afraid, characters who communicate obliquely rather than directly by exchanging poems, or dressing in a particular color, slipping a glance while participating in formal court occasions, all that.
This tale adds in spirits and gods and goddesses, possessed people and animals, a plot by a vengeful ghost. It very definitely moves quite a bit faster than Genji. But it still owes a lot to its literary forbear and some people may find the formal, ritualised Heian style interaction to be a little slow and frustrating. I didn't, I liked it. Very fun.