Death of a Ghost by M.C. Beaton

Retired police superintendent Handy Ebrington has bought a run-down castle in the town of Drim and wants the authorities to investigate, because he believes there's a ghost lurking about. Hamish Macbeth and his assistant, Charlie Carter, decide to spend the night in the castle and end up finding a dead body. Then the body disappears, and Hamish wonders if the crime might have been the work of smugglers. Soon, there are more murders, and Hamish attempts to wrap up the cases, all the while letting someone else take the credit (so he's not promoted and can stay in his beloved village of Lochdubh).