"Compendium of Materia Medica" is not a book.
Just like "Tianqi" is not just a medicine.
"Materia Medica" is an organization.
"Tian Qi" is a person in the organization.
Tian Qi is a killer.
So, this is a killer story, a story about two killers, and their friends.
Maybe it was just a game, a puzzle, a scam from the beginning...