in Editing
Endnotes, footnotes or neither in narrative non-fiction
Every source note is a small interruption. The question for narrative non-fiction is how to show your evidence without breaking the story.
Narrative non-fiction wants to be read like a story and trusted like a history. The notes are where those two aims meet. Get them wrong and the book either feels unreliable or reads like a thesis.
There are three broad choices, each with a long history, and none is correct for every book.
Footnotes
Footnotes sit at the bottom of the page, so the reader can glance down and back without losing their place. They are the most visible form of annotation, and that visibility is both the point and the problem.
They suit books where the notes carry content the reader will want in the moment: a qualification, a telling detail, a dry aside. Edward Gibbon's footnotes to The History of the Decline and Fall of the Roman Empire are famous partly for this, for being worth reading in their own right.
The cost is interruption. Every superscript number invites the eye to leave the sentence. In a book that depends on momentum, a page with several footnotes can feel like a conversation with someone who keeps checking their phone. They also complicate typesetting, since the notes must fit on the same page as the text they belong to.
Endnotes
Endnotes move the apparatus to the back of the book, or occasionally to the end of each chapter. The page stays clean, the story runs on, and the reader who wants the source can find it.
The drawback is that almost nobody flips back and forth while reading. Endnotes therefore work best when they are purely referential: sources, page numbers, archival references. Anything genuinely interesting placed in an endnote will be missed by most readers, so it belongs either in the text or nowhere.
Many narrative histories use a variant with no superscript numbers at all. The notes at the back are keyed to page numbers and a short phrase from the text. The reader sees no interruptions, and the evidence is still there for anyone checking. This approach does require care in production, since the page references must be finalised after typesetting.
Neither
Some books of narrative non-fiction have no notes, perhaps with a bibliography or a short essay on sources instead. This is a legitimate choice for some kinds of writing, such as memoir or reportage where the author's own observation is the main source.
It asks more of the reader's trust. Where a book makes contestable claims, quotes documents, or reconstructs scenes the author did not witness, readers and reviewers will reasonably want to know how the author knows. A book without notes has to earn that trust in other ways, often by saying in the text where information came from.
Choosing
A few questions settle most cases. How contestable are the book's claims? The more a reader might dispute, the more a note system earns its place. How much of the note material is interesting in itself? If a lot, footnotes or more text; if little, endnotes. Who will read the book? A general reader tolerates less apparatus than a specialist, but a specialist will look for it.
Whatever the choice, keep the system consistent across the book and decide early. Adding notes to a finished manuscript means revisiting every claim, and retrofitting a different system is slower still. The evidence should be gathered with the note form in mind from the first draft.
A good note system is one the reader barely thinks about until they need it.