A hundred years ago, Agatha Christie wrote her first mystery – The Mysterious Affair At Styles – and introduced the world to Hercule Poirot for the very first time.

The author went on to pen another 65 detective novels, 14 short story collections, six romances, and the world’s longest-running play, The Mousetrap.

She has sold over two billion copies of her books worldwide – one billion in the English language and one billion in over 100 different languages. Her books have never been out of print, and, more pertinently, she is the the first crime writer to have 100,000 copies of ten of her titles published by Penguin on the same day in 1948 - A Penguin Million.  

So it makes sense that the Royal Mail wanted to celebrate the legendary queen of crime’s career in style.

Read more: How Agatha Christie's mysterious disappearance resembles Gillian Flynn's hit novel, Gone Girl

“With more than two billion book sales worldwide, crime writer and playwright Agatha Christie has been outsold only by Shakespeare and the Bible,” said the Royal Mail.

“So it’s fitting that we mark not just the centenary of her writing her first crime novel, but also the 40th anniversary of her death with a stamp issue that’s full of intrigue.”

With a little help from designer Jim Sutherland, they created six stamps, each of which is devoted to a classic Christie novel.

However, unlike most stamps, these are extra-special; each contains a hidden clue, crafted with microtext, UV ink, and thermochronic ink.

Which means that, yes, you’ll need a magnifying glass, UV light, or body heat to reveal all of their secrets.

Read more: Iconic Agatha Christie cover art

“As ever – there’s so much detail in handcrafting for such small canvasses,” explained Sutherland.

“The books themselves are beautifully written with such clever plots and twists – we were keen to reflect that in the scenes chosen from each story.”

Each stamp also contains a hidden letter, which combines across the set to spell out a very special word…

Included in the set are:

• Murder on the Orient Express
• The Mysterious Affair at Styles
• The Murder of Robert Ackroyd
• A Murder is Announced
• The Body in the Library
• And Then There Were None

Read more: Kim Cattrall confirmed to star in BBC One's Agatha Christie thriller

There is also a a 3D animated version of the artwork on The Mysterious Affair at Styles stamp, for users who download the Aurasma app (via the iTunes App Store). Here, they can explore the scene being played out on the stamp, interact with the lighting, and take a direct route to the Royal Mail online shop. 

Speaking about the designs, Agatha Christie’s grandson, Mathew Prichard, said: “I am delighted that, 100 years after she wrote her first detective novel, my grandmother’s works are being celebrated in such a unique way by Royal Mail.

Read more: The cast for Agatha Christie's Crooked House is beyond incredible

"The ingenuity of her mysteries is cleverly represented in these distinctive designs, with the use of cutting edge technology - a welcome alliance of the traditional and the modern - adding to the delight. I am certain that Agatha Christie and mystery lovers will relish the puzzles that each stamp presents.” 

The stamps are available from 15 September via the Royal Mail website.