This change is imperceptible; but is, however, of the last consequence

David Hume in Treatise of Human Nature.

In every system of morality, which I have hitherto met with, I have always remark’d, that the author proceeds for some time in the ordinary way of reasoning, and establishes the being of a God, or makes observations concerning human affairs; when of a sudden I am surpriz’d to find, that instead of the usual copulations of propositions, is, and is not, I meet with no proposition that is not connected with an ought, or an ought not. This change is imperceptible; but is, however, of the last consequence. For as this ought, or ought not, expresses some new relation or affirmation, ’tis necessary that it shou’d be observ’d and explain’d; and at the same time that a reason should be given, for what seems altogether inconceivable, how this new relation can be a deduction from others, which are entirely different from it … [I] am persuaded, that a small attention [to this point] wou’d subvert all the vulgar systems of morality, and let us see, that the distinction of vice and virtue is not founded merely on the relations of objects, nor is perceiv’d by reason.”

"This change is imperceptible; but is, however, of the last consequence" - Rings true. I notice this most in news and magazine articles rather than in books (though I perhaps ought to be more alert).

Rarely is the article so unsophisticated as to use the terms "ought" or "ought not". Instead, they often move from a series of statements of fact to a series of unstated assumptions which actually hide the embedded "ought". If you agree with the assumptions, you never even notice. And if you don't agree with them, you are left wondering how you arrived at an indefensible conclusion from uncontested premises.

But reading Hume's passage, I was thinking of his point from a slightly different perspective. When dealing with "is", you are reporting facts as you understand them: in other words, non-fiction. However, when dealing with "ought" you are dealing with facts not as they are but as you would wish them to be. One might characterize such unproven desires as fictions. In which case, in reading the article, you are slowly led from a non-fiction reportage to a fictional piece. And you often don't notice it at all.

But think of characterizing a book as such, a book that goes from a non-fiction account to a fictional account. Are there any such out there? Non-fiction books with so much incorrect information that it feels to be fictional - sure. But I can't think of any book that is deliberately structured as a non-fiction book and then transitions without comment to fiction. It is an odd concept which, for some reason, I find intriguing.