The poison garden - Sarah Singleton

It is the 1850's, and a young boy, Thomas, leaves his family to be apprenticed to a pharmacist, at the behest of his dead grandmother. He also inherits a magical box from her, which provides him entry into a mysterious garden. But while visiting it, he sees a ghostly vision of his grandmother, who tells him she was poisoned, and warns him that he must find the person responsible, and save her precious garden. For she was one of five members of an arcane guild, each of whom cultivated an individual garden, mastering the art of poison, perfume and medicine. The guild members jostle for power as, one by one, they are murdered... can Thomas solve the mystery, before he in turn is threatened?