An early effort from Agatha Christie, featuring Hercule Poirot. A fix-up of a number of short stories, it's as much a thriller as a mystery. Not one of her best by any means and a bit cringeworthy, but acceptable. Plus, it's free. The best-formatted free editions I know of are available through Standard eBooks at standardebooks.org/ebooks/agatha-christie/the-big-four .