In my book
Definition 1 of 1
Pro Tip 1/3Subjective View
Expresses a personal view or preference, not a universal truth. 
Chocolate is the best dessert, in my book.
Pro Tip 2/3Figurative Speech
There is no actual 'book'; it's a figure of speech for personal belief. 
In my book, she's the team's most valuable player.
Pro Tip 3/3Informal Tone
Often used in casual conversation rather than formal writing or speeches. 
You're the best mechanic in town, in my book.