And Then There Were None by Agatha Christie is an iconic whodunit that lives up to its promise. The prose felt a little archaic, but the plot and mystery were compelling. If you are looking for a quick and enjoyable beach read, you won't go wrong with this mystery novel.
As an aside, I have decided to count this toward the Numbers Challenge. I believe it is within the rules because "None" is a number word in the same sense as "First". Also, this novel has also been published as Ten Little Indians, which would definitely qualify as a "Number" book.