When a book describes people and events that did not really happen, we call it “fiction”.
When a book has no such descriptions, we call it “non-fiction”. We do not call it “fact”.
A book may contain things that are not descriptions of people and events, but, for example, analysis, argument, conjecture, dogma, theory, politics, morality, logic, mathematics, experience, life histories, explanations, instructions, justifications, exhortations, prophesies, speeches (in which the reporting of the fact of the speech may override consideration of the validity of what was said in the speech), praise, seduction and so on.
Yet when we distinguish fiction from non-fiction, we are only concerned with people and events.
And only some of the events, for every work of fiction contains some facts, often a complete factual social milieu and surrounding history; at the very minimum, that there is a universe and creatures capable of perceiving the story being told.
We do not make “fact” the opposite of fiction because it is obvious that most non-fiction contains quite a lot that falls short of undeniable fact. Even a trained and meticulous historian could not produce, say, a history of the US wars in Middle East, that would be accepted by all people, or even half of people.
Yet fiction is used to tell facts about people, relationships and society that are often close to universally acknowledged. No-one who has read Hanya Yanagihara ’s A Little Life would doubt the impact that being sexually abused as a child can have on an entire life. No-one who has read Wally Lamb’s The Hour I First Believed could deny that our morally defective legal system unfairly damages people’s lives.
Both of these subjects are matters of factual debate and political governance, yet the dept of emotional understanding available to the reader is far greater than any text or report could achieve.
Even bad books, or books with cardboard characters, tell us things about how humans think and act. Often they unintentionally reveal a depth of fact about the author’s mind and soul. Meaning that was not consciously intended is a hallmark of art.
Fiction helps us understand ourselves and our world and that is a matter of fact.
I claim (as a fact) that if you top the top 50% of all published fiction, it would contain more reliable fact about people, relationships and society than the bottom 50% of all published non-fiction.
The standard of proof adopted will affect an assessment of my claim. Let’s stick with the most objective thing we have, science. 96% of climate scientist agree that human actions are pushing global temperatures up. But this is not enough to create a “fact”. Those who reject the science include the presidents and prime ministers of powerful and well-educated countries.
Perhaps I should claim that there is simply more fact in fiction than non-fiction and not even qualify the assertion.
Even the thing which most distinguishes fiction from non-fiction (made-up stuff) is suspect. The Christian Bible contains many people and events the authenticity of which can be challenged. A majority of Christians see most Bible stories as allegorical (ie, made up) but I doubt there is a Western publisher willing to list the Bible under “Fiction”.
Perhaps I should go further and ask, is there anything that is a universal fact, outside of fiction?
Anyway, all I can say is, if you want to know something important about being human and understanding ourselves, try a novel, not a “non-fiction” book.