Shimmer by Alyson Noel (author)

Having solved the matter of the Radiant Boy, Riley and her companions are enjoying a well-deserved break. But then Riley comes across a vicious black dog, and decides to cross him over. Following the dog, she runs into a young ghost named Rebecca. Despite Rebecca’s sweet appearance, Riley soon learns she’s not what she seems. The daughter of a former plantation owner, she is still furious about being murdered during a slave revolt in 1733. Her uncontrollable anger is keeping other ghosts, who also died in the revolt, trapped in a shimmering bubble. And when Bodhi and Buttercup fall into Rebecca’s trap, Riley is the only one who can save them…