Julia Golding's Mines of the Minotaur is the third in the Companions Quartet.
Connie is almost unique. She is a Universal, which means she has immense power. The problem Connie has is that she doesn't really know how to use her power, and neither does anyone else in the Society for the Protection of Mythical Beasts because it's been so long since they had a Universal in their midst. Only one person understands Connie's potential and that's the evil shapeshifter, Kullervo. What Connie doesn't know is that he has started infiltrating her dreams, causing her to sleepwalk and raise a devastating storm. And when she's awake, the darker side of Connie's power is beginning to feel increasingly attractive, especially if she's angry or upset. When her best friend, Col, discovers what's happening, he begs Connie to seek help, so she turns to the Society, the group of people and mythical beings who are meant to protect her. But they're so terrified by what she tells them that they expel her. Feeling abandoned and scared, Connie visits an old abandoned mine where she finds she isn't alone as the mine's full of sick mythical creatures that the Society has forgotten. They are led by the once-proud Minotaur who, having been blinded in a fight, is now a broken, nameless creature. However, he gives Connie the courage to face her dark side, and in so doing begins to heal himself. I like this series, but the books still seem very didactic, especially on the issue of environmentalism. Somehow it feels too in-my-face (which I find distracting), moreso than the anti-slavery themes of the Cat Royal series.
The first two books in the Companions Quartet are The Secret of the Sirens and The Gorgon's Gaze. Julia Golding seems to have turned into a one-woman story-telling phenomenon lately as she's also published Den of Thieves, the third in her Cat Royal series, The Ship Between the Worlds a stand-alone pirate tale, and in June Ringmaster, an African adventure story, is out. I'm hoping to get both Den of Thieves and The Ship Between the Worlds from the library soon.