“I am not one to rely upon the expert procedure. It is the psychology I seek, not the fingerprint or the cigarette ash.” — Hercule Poirot in Murder on the Orient Express

When choosing which books to pull off the shelf to take on summer vacation, any beach bag, lake tote, or mountain satchel would be sadly lacking not to include at least one cozy murder mystery, as equally suited to the warmth of summer sand as to that of a winter fire. For this delicious genre, we largely have “The Queen of Crime” to thank. Outsold only by the Bible and Shakespeare, the third most-read author of all time Dame Agatha Christie largely crafted the structure we now expect in a modern detective novel.

Today, her books have more than 2 billion copies in print across 45 languages, including 66 novels and more than 150 short stories, all of which were originally published between 1920 and 1976, now referred to as “The Golden Age of Detective Fiction.” She also wrote the world’s longest-running play, the murder mystery The Mousetrap, and her stories have seen a resurgence of popularity in Hollywood lately with Kenneth Branagh’s film adaptations of Murder on the Orient Express and Death on the Nile.

While Wilkie Collins invented the genre of detective fiction with The Woman in White (1859) and The Moonstone (1868), and Sir Arthur Conan Doyle popularized it with his Sherlock Holmes stories in the 1890s, Christie, highly influenced by both of these writers, established the guidelines we now anticipate in a good “whodunit.” E.g., a story of no more than 10 or so characters, all of whom become suspects after one unexpectedly — to them — drops dead. A detective then arrives on the scene, be it a professional or merely an observant, fluffy-haired old lady, and begins to collect the facts and assemble them, culminating in the big reveal during which the reader usually has a few opportunities to make a final guess.

Christie and several other notable writers of detective fiction, including G.K. Chesterton, Ronald Knox, and Baroness Orczy, met in 1930 to form the Detection Club. At one of their dinners, they decided on what are now termed the “rules of fair play,” meant to guarantee the reader all of the information available to the detective and therefore a fair chance at solving the murder by the conclusion … rules which Sherlock Holmes routinely did not observe. “The 10 Commandments of Detective Fiction” are as follows:

1. The criminal must be someone mentioned in the early part of the story, but must not be anyone whose thoughts the reader has been allowed to follow.

2. All supernatural or preternatural agencies are ruled out as a matter of course.

3. Not more than one secret room or passage is allowable.

4. No hitherto undiscovered poisons may be used, nor any appliance which will need a long scientific explanation at the end.

5. No Chinaman must figure in the story (an attempt to curtail the popular, and rather racist, device of the time of a stereotypical “Chinese mastermind,” which had become a cliché culprit).

6. No accident must ever help the detective, nor must he ever have an unaccountable intuition which proves to be right (ahem, Holmes).

7. The detective must not himself commit the crime.

8. The detective must not light on any clues which are not instantly produced for the inspection of the reader (ahem, ahem … looking at you, Holmes).

9. The stupid friend of the detective, the Watson, must not conceal any thoughts which pass through his mind; his intelligence must be slightly, but very slightly, below that of the average reader.

10. Twin brothers, and doubles generally, must not appear unless we have been duly prepared for them.

They also swore an oath: “Do you promise that your detectives shall well and truly detect the crimes presented to them using those wits which it may please you to bestow upon them and not placing reliance on nor making use of Divine Revelation, Feminine Intuition, Mumbo Jumbo, Jiggery-Pokery, Coincidence, or Act of God?”

As many would argue, there are occasions where rules are meant to be broken, and The Murder of Roger Ackroyd breaks one of these rules rather powerfully, which actually makes it my favorite Christie mystery. I am apparently not alone in this sentiment as the 600 members of the Crime Writers’ Association chose it as “the best whodunit ever written” in 2013. It is interesting, however, to consider which are Christie’s favorites.

In her introduction to Crooked House, Christie writes, “This book is one of my own special favourites. I saved it up for years, thinking about it, working it out, saying to myself: ‘One day, when I’ve plenty of time, and want to really enjoy myself — I’ll begin it!’ … Crooked House was pure pleasure. I often wonder whether people who read a book can know if it has been hard work or a pleasure to write?” I certainly found her pleasure in this one evident and thoroughly enjoyed each character, struggling to land on a probable culprit.

For the avid Agatha Christie enthusiast, Harper Collins recently published a complete volume of all the short stories involving Belgian detective Hercule Poirot, self-designated as “probably the greatest detective in the world.” Although lacking in humility, his distinctive mannerisms, bubbly personality, and dapper presentation all serve to create a truly lovable character. For those who have already exhausted his extensive collection of novels, this short story collection is the perfect remedy to continue on his adventures.

Happy guessing!