in one's book

Refers to a hypothetical book or list of one's opinions and beliefs.
Slang from the mid-1900s.[1]

in one's book

1. (slang) In one's opinion.
   • 1973, QST, Volume 57, Issue 2, page 87,

     Is this "the field?" Not in our book, it isn't.

   • 1975 Feb 8, Kevin O'Donohue, Stan Clark, Brian McClenaughan, Frank Jeffcoat, Four Broadcasters Compare U.S. Radio with Australian Radio - The Knockout Blow, in Billboard, page A-7,

     [Jeffcoat:] I'll qualify that in a moment, but in my book I think when it comes to sheer professionalism, there's nothing quite like an American broadcaster.

   • 1978, William James, Frederick Burkhardt, Fredson Bowers, Ignas K. Skrupskelis, Essays in Philosophy, page 66,

     My solution, or rather Stumpf's (for in my book I am but the humble follower of the eminent Munich psychologist), was to take neither of these objectionable alternatives, […] .

   • 2007, Ralph A. Gessner, Deep in My Heart, page 105,

     That's taking things way too far in my book.

2. Used other than with a figurative or idiomatic meaning: see in,‎ one's,‎ book.

• Only the pronoun form one's changes to match the subject of the phrase.