An excellent mystery from the queen of the "had I but known" genre. Keeps you guessing till the end.
Before Agatha Christie, before Dorothy L. Sayers, there was Mary Roberts Rinehart. And, frankly, her mystery novels are a great deal more realistic (in a fantastical way) than her successors. Too bad that her work is largely unsung today, except maybe at Gutenberg.com if you are into that. The unanswered questions build throughout, culminating in rather simple, yet plausible explanation at the end.
Mary Roberts Rinehart, just one of the finest mystery writers in all time. She is called the America's mistress of mystery. You will love this book, just try falling alseep reading it.