Dandy Gilver and a Spot of Toil and Trouble (Book 12) UK.

Fair is foul and foul is fair when aristocratic private detective Dandy Gilver arrives at Castle Bewer, at midsummer 1934, to solve the tangled mystery of a missing man, a lost ruby and a family curse. The Bewer family's latest wheeze to keep the wolf from the door is turning the castle keep into a theatre. While a motley band of players rehearse Macbeth, the Bewers themselves prepare lectures, their faithful servants set up a tearoom, and the guest wings fill with rich American ladies seeking culture. Meanwhile, Dandy and her sidekick Alec Osborne begin to unravel the many secrets of the Bewers and find that, despite the witches, murders and ghosts onstage, it's behind the scenes where the darkest deeds are done.