Rhianna and the Castle of Avalon — Dave Luckett

The shelf of books before them and the wall itself rose smoothly from the floor; straight up into the ceiling until a narrow doorway the height of a tall man was revealed. It was the top of a staircase, and the steps curved away into the darkness, downwards and out of sight... Wild Magic is loose in the kingdom, and the Queen needs Rhianna at the Castle of Avalon. But the castle is full of secret passageways and hidden tunnels, and evil is closer than anyone thinks.