I don't know if this story was supposed to be mystery, but it's not my idea of one. I expect to be kept guessing till the end, and I expect to witness the investigation and discovery of the Big Secret. In this book, the Big Secret is revealed early, and we see none of the work of its discovery. It's just announced to us. The remaining answers are revealed in flashbacks, or in the perpetrators' private thoughts. (We don't even get an "And the secret will die with you! BWA-HA-HA-HA-HA
!") Altogether, not satisfying to me.