Conflict of Interest Disclosure
A conflict of interest disclosure is the statement, attached to a piece of research, in which an author tells the reader what they stand to gain from the result. Journals require it. Trade publishers do not. That asymmetry is the whole content of this concept, and it produces a consequence nobody designed and nobody polices: the same author can disclose an interest in a paper and omit it from the book that popularises the paper, without breaking any rule.
What it is
The requirement attaches to the venue rather than to the person or to the claim. So it travels with the least-read version of the work and falls away from the most-read one. A trade book is therefore the venue where a commercial interest is most consequential to the reader and least likely to be declared.
Nothing in this concept is an allegation of misconduct, and it should never be used as one. Having a commercial interest is not a wrong in itself; it is the ordinary outcome of a scientist whose work reaches a clinic. Authors are not obliged to supply disclosures nobody requires. The object of the criticism is a convention's scope.
The rule that follows is practical. Before citing a trade book's recommendation of any intervention, look up whether the author has a relationship to it, and say so in your own copy whether or not the book did. The check takes a minute and the book will not do it for you.