The Idea Factory - Weapons of Mass Cuteness
This is probably more of a short story than a book. It starts out at a black market sale type of situation, with the main character buying something that isn't revealed to the reader at first. At the end of the scene, they take out the item and it's a kitten. They reveal that this is a big thing that's been happening lately, people selling kittens on the black market. But these aren't normal kittens. It eventually is revealed that the kittens are really weapons, blowing up somehow or just causing chaos in some way with their general cuteness.