Wedgewood (Bk 8) - Silver Sagas, #8

An intertribal treaty negotiated eons ago forbids the four surface tribes from publicly acknowledging the existence of the Water Fairy Tribe or suffer its wrath. Over time, even the stories have faded until only the royal families remember and guard against this danger. These circumstances severely complicate matters for Lady Cassidy of the Water Fairy Tribe, stranded for the winter in the Wood Fairy city of Weetu.
In addition to her volunteer medical duties, she must constantly watch what she says and does or risk revealing her identity. In her struggle to keep her tribal secrets, Lady Cassidy keeps everyone in Weetu at arm's length-even Isaac, the attractive Wood Fairy Crown Prince she's trying so hard not to love.