The premise is that a spy has put the books shown here in a prominent position to send a hidden message to a confederate.
To the person in the know, what you can see in the image tells when a meeting will take place. But when is it?
Unlike a conventional cipher, the requirement here is not just to work out the relationship between one set of characters and another - you need to find the message in the first place.
This kind of puzzle is an aid to creative thinking - you need to look for ways that the book spines can convey information and decide how to extract the required information from surroundings that are simply there to distract.
The message is concealed in a systematic way - it is not random. You are looking for a single word.
As a small incentive, I will send a signed copy of my brand new book Game Theory to one of the correct entrants who send in a solution by 10 April.
You can see the image bigger by clicking on it - if you'd like to enter your solution, go to the bonus puzzle page on the Conundrum website.