Well, I think it’s a bit of a stretch to call these mysteries.  They’re word problems. They’re good word problems (real-world situations, all the necessary information provided, but leave the solver to figure out what to do with the information, well explained), but still, they’re word problems.  Twelve story problems are put forth, each on it’s own page, and on the reverse of that page the answer is preceded by an explanation of one way to solve the problem.  It does what it’s trying to do well, so if you’re looking to fill a niche like this, I could recommend this title, but I don’t imagine it’s likely to have a lot of circulation, as it is unlikely students will self-select it, and most teachers already have an arsenal of materials they draw from.  Also, I do wish they would have made more of a point of stating that the solutions offered represent one way to think about the situation, rather than just presenting it as THE solution.