Investigator is a type of Expert. Investigator is related to Agent. Agent is related to Secret. Secret is a type of Book. 
Stranger is a type of Book. Broker is a type of Book. 
Expert <--IsA-- Investigator <--RelatedTo--> Agent <--RelatedTo--> Secret --IsA--> Book 
Stranger --IsA--> Book <--IsA-- Broker 