The Mysterious Affair at Styles

Agatha Christie's debut novel was also the first to feature Hercule Poirot, her famously eccentric Belgian detective.
A refugee of the Great War, Poirot has settled in England near Styles Court, the country estate of his wealthy benefactor, the elderly Emily Inglethorp. When Emily is poisoned and the authorities are baffled, Poirot puts his prodigious sleuthing skills to work. Suspects are plentiful, including the victim’s much younger husband, her resentful stepsons, her longtime hired companion, a young family friend working as a nurse, and a London specialist on poisons who just happens to be visiting the nearby village.
All of them have secrets they are desperate to keep, but none can outwit Poirot as he navigates the ingenious red herrings and plot twists that contribute to Agatha Christie's well-deserved reputation as the queen of mystery.
Librarian's note: the first five novels in the Hercule Poirot series are #1) The Mysterious Affair at Styles, 1920; #2) The Murder on the Links, 1923; #3) The Murder of Roger Ackroyd, 1925; 4) The Big Four, 1927; and #5) The Mystery of the Blue Train. Note that there are many short stories and collections in this period. Each novel and short story has its own entry on Goodreads.