"Which" is used to refer to something previously mentioned. For example, "I have to write a report by tomorrow for 'Crime and Punishment,' which I haven't even read yet." In this example, "which" is used to refer to the book "Crime and Punishment." So, "in which" means "in the previously mentioned noun." For example "'Crime and Punishment' is a book in which..." Basically means "In 'Crime and Punishment....'" Hope that helps.