The Hidden Book

It's Victorian London, and the air is thick with fear: a spate of poisonings have been reported across the city. Every victim is different, but each crime has one thing in common: an empty vial is found by the body. There is only one suspect: the old shopkeeper of the magic shop on Drury Lane. With the shop closed and no sign of the owner, you risk entry late at night to reveal the truth behind the crimes. Entering the magic shop, it looks undisturbed. You'd suspect nothing, were it not for the missing book which usually lies open on the counter. Where is this missing book? What did it contain? And does it prove that the old magician is the perpetrator or, in fact, completely innocent?