And Then There Were 23: After the Funeral by Agatha Christie

We both adored this (somewhat) unsung Christie gem... though not
without reservations. Fortunately, for purposes of discussion,
those reservations turned out to be slightly different. It's a
truth universally acknowledged that every unhappy family is unhappy
in its own way, and that certainly holds true for the various
familial arrangements portrayed so vividly in this sprawling novel.