The Forbidden Land (The Witches of Eileanan Book 4) By Kate Forsyth

Finn is bored finding the life of a Banprionnsa tedious and is overjoyed when Dide and his family of Jongleurs come with a mission for her from the Righ. To go deep into the Forbidden Land of Tirsoilleir and break the Prophet Killian the Listener out of the Black Tower his unassailable prison in the hopes that he can raise the people of Tirsoilleir against their rulers.

This is definitely my favourite book of the series. Even though Finn can be a bit petulant she is fun and enjoys her adventure to the fullest. And even usually unlikeable characters like Lachlan do not show there irritating characteristics.