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.'

Question

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.'

Answer

"The oldest trick in the book" is an expression used to describe something clever that people have been doing for years. It has the connotation that it still works well despite having been around for a very long time.

As speakers, we can play around with the meaning of this expression by substituting the word "trick" for something else. For example: "the oldest recipe in the book" or "the oldest method in the book". It maintains the same basic connotation though.