A few months ago, the U.K.’s Royal Mail released a set of stamps inspired by the works of Agatha Christie, one of the greatest mystery writers of all time, in honor of the 100th anniversary of her first detective story – The Mysterious Affair at Styles, which introduced Hercule Poirot, the famous fictional Belgian detective. But these are no ordinary stamps.

Each of the six designs is based on one of Christie’s popular novels and each contains hidden clues pertaining to its respective title — Murder on the Orient Express, The Body in the Library, And Then There Were None, The Murder of Roger Ackroyd, A Murder is Announced, and the aforementioned The Mysterious Affair at Styles.

Check out images of all six designs here below.

The clues, which are embedded into the stamps using microtext as well as heat-sensitive and UV inks, can be uncovered by using UV light, body heat, or a magnifying glass. One of the designs, for Styles, also includes an augmented reality feature that triggers a 3D animation when used with a downloadable app.

Designed by Jim Sutherland of the London-based Studio Sutherland in collaboration with British illustrator Neil Webb, each stamp has clues that point to the murderer of the story, as well as a hidden letter that when all are combined spells out “Agatha.”

This Agatha Christie stamp set, which is also available in a Presentation Pack written by Agatha Christie’s grandson Mathew Prichard, is a delight for fans, philatelists, and anyone who enjoys such ingenuity. There’s also postcard and First Day envelope sets.

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[Source: Creative Review via Slate]