The Underground Ghosts by Dori Butler
It's almost Halloween, and Kaz and Claire are off to Seattle to visit Claire's cousins. They explore the local library . . . and find a ghost living there! The ghost's family has been missing, so Kaz and Claire decide to investigate. When they discover ghostly activity in the city's subterranean passages, they put their detective skills to work. Kaz and Claire are on the case!