There's a reason this book is often posited as Christie's masterpiece, & it's not (or perhaps not just) the astoundingly good twist at the end. Throughout the narrative she leaves a commendable number of Checkov's guns; every last one of them is resolved. 
 She uses setting and character beautifully, and her style is, as usual, extremely readable. 
 A genuine, unabashed five stars for this one.