Thirteen Days of Midnight Orchard books

When Luke Manchett's estranged father dies suddenly, he leaves his son a dark inheritance. Luke has been left in charge of his father's ghost collection: eight restless spirits. They want revenge for their long enslavement, and in the absence of the father, they're more than happy to take his son. It isn't fair, but you try and reason with the vengeful dead. Halloween, the night when the ghosts reach the height of their power, is fast approaching. With the help of school witchlet Elza Moss, and his cowardly dog Ham, Luke has just thirteen days to uncover the closely guarded secrets of black magic, and send the unquiet spirits to their eternal rest. The alternative doesn't bear thinking about.