in one's book

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

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in one's book

1. (informal) In one's opinion.
   • 1973, QST, volume 57, number 2, page 87:

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

   • 1975 February 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.

   • 2012 October 6, Ed Vulliamy, “BB King at 87: the last of the great bluesmen”, in The Observer‎:

     Around 11pm BB King appears onstage, much of his audience one over the eight, talkative but mellow, ready for what is (in my book at least) the experience of a lifetime.

2. Used other than figuratively or idiomatically: see in,‎ one's,‎ book.

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