in somebody's book

in (one's) book

According to one's opinion, judgment, or code of personal ethics. In my book, it's worse to run away from a fight than to start one. You kissed a girl, even though you have a girlfriend. That's cheating in my book, dude.
See also: book

in somebody’s book

(spoken) in somebody’s opinion; according to somebody’s judgement: They took the car away without asking me, and in my book that’s theft.
See also: book