in one's book

Refers to a notional 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.

   Near-synonyms: in one's view, in one's eyes, to one's mind; see also Thesaurus:in my opinion

   Clara did nothing wrong in my book.

   In my book, there's nothing wrong with that.

   In his book, that counts as an affront, for some reason.

   • 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^[1]:

     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.

   • 2025 November 12, Philip Haigh, “High praise for rail staff's courage and clear thinking”, in RAIL, number 1048, page 50:

     Huntingdon's ferocity makes the reaction of staff all the more brave. To move towards danger when others are fleeing is the definition of bravery in my book. To place yourself in danger to protect others.

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

   Today's homework is in your book. Could I borrow it?

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