The Mysterious Affair at Styles by Agatha Christie
The book begins at Styles Court, located in Essex, UK. The mansion has several people living in it, including Emily Inglethorp, the elderly owner of the estate. When the rest of the people who live there wake up in the morning, they find that Emily has died and it looks like she was poisoned.
In one of Christie's novels about Hercule Poirot, the famous detective discovers that Emily had great wealth, providing a highly probable motive for the murder. The suspects included Alfred Inglethorp, a younger man Emily just married, the Cavendish family, which includes Emily's stepchildren, and Cynthia and Evelyn, both Emily's friends.
John Cavendish, according to Emily's will, is the one who will get the majority stake in the mansion, so at first, this points the suspicious finger at her. However, it is said that, on the day of the murder, Emily had argued with John and Alfred and changed him will. No one can find any evidence that such a newly created will exists.
The crime investigator is Inspector Japp, who believes that Alfred is the one who committed the murder. He refuses to provide an alibi, and he's the one with the most to gain (they found an updated version of Emily's will, in which Alfred inherits the mansion). Because he is so young, it seems that he is simply a man looking for easy money.
Alfred is found guilty of the murder and he did it in coordination with Evelyn. Apparently, he had written a letter about Emily's murder, and this is what made Emily so angry on the day of the murder. Not being able to go another day without her telling someone, he decided to kill her that night.
