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?