What does 'in the book' mean?
The full sentence:
So your unconscious has found a very colorful way of painting one of the oldest ones in the book.
The context is a man has been dreaming about fXXking his dad, and he ask the therapist what his dream means. The therapist says' many people struggle with the concept of wanting to make their parents proud, it's genetic from birth. We need their affection and their approval to survive SO YOUR UNCONSCIOUS HAS FOUND A VERY COLORFUL WAY OF PAINTING ONE OF THE OLDEST ONES IN THE BOOK.'