Sometimes I read non-fiction history and those authors can really get carried away with footnotes, to the point where the footnotes occupy more space on the page than the text above.
I looked up 'when to use footnotes' and got this:
"You should always use footnotes (or endnotes) in order to give the source of facts or opinions which you have obtained from outside sources. If you quote any author, or document of any kind, you must specify where the original information can be found. This is necessary for one reason only: your reader may want to consult the same text or document, for whatever reason, and so you must specify it.
Of course, footnotes may also contain textual material of your own doing. There are times when you want to make a statement about something but it doesn't quite fit in with your outline. The best thing to do is to include this information as a textual footnote. In this way, you can still include the information without taking away from the flow of the essay."
So those historians were following the above rule. Fine. But the non-fiction book I'm currently reading is getting on my nerves because the footnotes seem to me to be as much in the flow of the story as the story itself. I tried ignoring them, then I tried reading them when I got to the bottom of the page, but had to go back to put them into context.
For example, reading about the tragic sinking of the ironclad ship Victoria off the coast of Tripoli in 1893:
"On the other hand Brackenbury, in the unwieldy Edinburgh, who had started to turn a little beforehand in order to off-set his ship's sluggishness, and who was swinging rapidly towards the motionless Camperdown, reversed his helm very promptly and passed outside her. (1)
Bottom of the page:
(1) This goes far to support the view that the Victoria and Camperdown might have avoided collision, even when end-on to each other."
Now why couldn't he have put that footnote thought in the text? A little comma, maybe an occasional semi-colon would have gone a long way to keep the narrative moving along. I'm going to lose patience with this guy and have to pick up a Perry Mason or Miss Marple for some relief.

Cataract Canyon from Dead Horse Overlook (near Moab).