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The idea is simple: we intend to publish a puzzle book that contains a difficult enigma. The first one to solve this enigma will win a prize (i.e., $10.000). Some obvious rules:

• The solution needs to be validated by us (the authors);


• In order to be the first we are going to provide the means for proving
  it (an e-mail address);


• After the prize is given, we need to publish somewhere (a website) that it has expired and we are not accepting new submissions.

My question is if we are the first to propose something like this, or if there are other books like this. I have found only treasure hunting books and stories like the Goonies, but nothing promising real prizes.

As a bonus: is there any law-esque problem we are not seeing?

There have been several similar books in the past, known as "armchair treasure hunts". However, these generally involve actual physical treasures that have been buried somewhere, and cracking the riddles in the book leads you to their location. The principle is the same, though: the author of the puzzle has to confirm that your solution is correct before you can go off and dig up the prize. In your case, you'd confirm that the solution is correct and then simply hand over the cash.

I don't think there would be any legal issues - you certainly won't have to deal with treasure-seekers going around and digging up people's back gardens because they think that's where the prize is. The main issue is the possibility that nobody will ever actually crack the code and claim the prize - On The Trail of the Golden Owl, for example, hadn't been solved by the time the author died in 2004 and remains unsolved to this day.

Legal issues depend on where you live and who your contestants are.

In the US, there is a difference between sweepstakes, which are games of chance, and contests, which are games of skill. Your proposal is for a contest. Inc has written an excellent article outlining the various issues.

For a prize as substantial as $10,000, I would hire a lawyer to go over and improve your contest rules. $500 can save you a world of pain later (including things like perhaps having to award a duplicate prize).

Have a central location to make announcements.

A website is the best because then you can print the full rules, where to get a copy of the question/book, etc. Use Twitter to point to the website every time you update it. And to send reminders, etc. You could also use Facebook but Twitter would be a more reliable choice as it's public and they don't restrict who posts go to like Facebook does. State in your rules that it is the contestants' responsibility to keep up with the website if there are changes.

I was given a book which offered a cash prize to the first person who solved the puzzle. Because the time limit for claiming the prize had already expired and the book was not very engaging, I didn't bother even though I quite like solving puzzles. This was nearly thirty years ago and so the concept is not new.