Lost Treasure of Ytir

This is the last in a fantasy trilogy which began with Like Father, Like Daughter, and continued Arian Malcolm's adventures in Quarter Day. It takes place partly in the book town of Hay-on-Wye, in this world, and partly in the fantasy world of Ytir and the Six Duchies.
Arian Malcolm is sent by the Ard Ri of Ytir to find out whether the rumours that a Lost Treasure of Ytir has been discovered at a sacred spring in the Segur Forest are true. But others have heard the rumours, too.... Arian, her father Mal Petroc and her adopted sister Branwen get caught up in a plot to overthrow the Ard Ri, a plot that comes to involve a werewolf and his wolf pack, a dragon and a unicorn horn.