What is the meaning of sentence -To be in good books?

The phrase "to be in someone's good books" means to be in someone's favor or on good terms with them. It implies that the person is happy with you and approves of your actions.

Example:

* "I tried to be on my best behavior so I could be in the boss's good books."

* "He's been working hard to get back in his girlfriend's good books after their fight."

The phrase is an idiom, meaning its meaning is not literal. It's often used in informal speech and writing.