The Haunted Library — Dori Hillestad Butler

When ghost boy Kaz's haunt is torn down and he is separated from his ghost family, he meets a real girl named Claire, who lives above the town library with her parents and her grandmother. Claire has a special ability to see ghosts when other humans cannot and she and Kaz quickly form a friendship. The two join forces to solve the mystery of the ghost that's haunting the library. Could it be one of Kaz's lost family members?