Mary Roberts Rinehart 
The Circular Staircase [EPUB ebook] 

The orphaned niece and nephew had been under the care of Rachel Innes, a spinster, since they were little. Rachel is persuaded to rent a summer home in the country by her siblings Halsey and Gertrude, who are now 20 and 24, respectively. Thomas, a senior employee who has long served the Armstrong’s, gives them the alarming warning that there is a ghost in the house.
Even though there are still break-ins and strange tapping noises during the night, Rachel is determined to stay in the house until the mystery is solved and her nephew is proven innocent. She learns that Thomas and Mary had been concealing a very ill Louise Armstrong—the fiancé of Halsey and the stepdaughter of Paul Armstrong—in the groundskeeper’s home.
Rachel manages to shoot the would-be-burglary in the foot but the burglar still gets away. Against her will, Louise breaks off her engagement to Halsey in order to wed the town coroner. In order to aid in the inquiry, Jack Bailey pretended to be the new gardener. He finally cleared himself of all allegations, and he later married Gertrude. The housekeeper Mary, who was also the sister of Arnold Armstrong’s unfaithful ex-wife, shot Arnold Armstrong.