The Great Mistake by Mary Roberts Rinehart

Trade Paperback.

In an elaborate house known as the Cloisters, Maud Wainwright rules supreme. The queen of society in the small town of Beverly, she has a table long enough to seat one hundred, and she keeps an iron grip on the guest list. Her right-hand woman is Pat Abbott, a local girl who is beautiful, innocent, and kind. Pat has no idea how cutthroat high society can be, but she's about to get a deadly first lesson.

Pat has fallen head over heels in love with Maud's son, Tony, a clever young rake with a single flaw: his vicious, gold-digging wife. At the same time that she is dangerously infatuated with a married man, Pat's world is turned upside down by a series of attacks on the estate--and a truly shocking murder. To save Tony and Maud, Pat must find the killer. But the list of suspects is as long as one of Maud's guest lists: When a woman has room at her table for one hundred friends, she'll have more than her share of enemies.

Atmospheric writing, period detail, and characters caught in an intriguing murder plot make this Golden Age mystery one of Rinehart's best, exemplifying why she's known to her fans as "the American Agatha Christie."