Sometimes, when you are doing main story missions, you will see missions that need you to approve books to your council members. Have you saw that whenever you recruit a new council member-- to name a few approaches-- you will get a lot of purple books? Those are Baijia Congshu, and these are the books the game is speaking about. These books will bestow a council member with random stats, so make certain to use them to power up your preferred members.