The Mysterious Affair at Styles by Agatha Christie

The Mysterious Affair at Styles is a detective novel by Agatha Christie, written in 1916 and was first published in October 1920.
The Mysterious Affair at Styles was Agatha Christie's first published novel, introducing Hercule Poirot, Inspector Japp, and Lieutenant Hastings. The Mysterious Affair at Styles is told in first person by Hastings and features many of the elements that have become icons of the Golden Age of Detective Fiction, largely due to Agatha Christie's influence. The story is set in a large, isolated country manor. There are a half-dozen suspects, most of whom are hiding facts about themselves.
Upon her husband's death, the wealthy widow, Emily Cavendish, inherited a life estate in Styles as well as the outright inheritance of the larger part of the late Mr. Cavendish's income. Mrs. Cavendish became Mrs. Inglethorp upon her recent remarriage to a much younger man, Alfred Inglethorp. Emily's two stepsons, John and Lawrence Cavendish, as well as John's wife Mary and several other people, also live at Styles. John Cavendish is the vested remainderman of Styles; that is, the property will pass to him automatically upon his stepmother's decease, as per his late father's will. The income left to Mrs Inglethorp by her late husband would be distributed as per Mrs. Inglethorp's own will.
Late one night, the residents of Styles wake to find Emily Inglethorp dying of what proves to be strychnine poisoning... (Summary by Wikipedia)