In non-fiction, readers sometimes skip the footnotes and come back and read them when they finish the chapter. In fact, when footnotes are endnotes at the end of the book instead of chapter, I frequently just read them when I get to the end of the book. In non-fiction, you can do that. Footnotes do not add anything to a non-fiction story except more background. If you are writing a fiction book and using footnotes like this, it will fall flat.
You see, footnotes in fiction should add to the story - and not everyone can do that well. Many authors, famous authors, have tried and not fared well. Footnotes, by nature, break up the story. If you want to explain something, do it in text without diverting the reader down a side path. However, footnotes can work very well when they are part of the story. Still, I do not recommend them.
Most of the manuscripts I edit come from authors who have had little professional (university level) writing education. They majored in business, nursing, education, or didn't even attend college at all. Some of them don't even read the genre of books they are writing (or read any books at all).
Please note: If you want to be a professional writer, you at least HAVE to read or hire someone who does read to completely ghostwrite your book. If you do not read, you have no clue about plotting or how novels work, and quite frankly, you will end up sticking a glossary in the center of the book and switching from civil war south to gypsies in Europe halfway through. Yes, you can be self-published, but no, you should not be. Harsh, perhaps, but truthful. Would you want a doctor working on you who didn't read medical journals to keep up with the latest developments in medicine? Would you want a mechanic working on your car if he didn't even own one car of his own? On the road, do you want to be anywhere near the driver who has never driven a car before?As you can see, I do my own "footnotes" on my blog. They do break up the text, and you can come back and read them later. Despite the fact that I know how to use footnotes (although don't base your opinion of that on the way I use them in my blog), I would not want to insert them into fiction. You have to be completely immersed in the fantasy world of your book to make people happy about reading them.
The authors I work with are not. They have done a lot of research and want to share bits of information they learned. This can be done just as well by adding the information into the story subtly and discriminately. The reader should be able to know that you did a good job with your research just from reading it. At the same time, the reader does not need to know everything you researched.
This is true of non-fiction, too. When I write a scientific paper, lets say I look up 20 - 40 papers on the topic. I read through the abstract and determine which ones apply to my topic specifically. The abstract may weed out 10 - 20 that I do not have to read. As I read the rest of them in full, I again weed out some that are not quite addressing my topic. In the end, I may have 5 - 10 papers listed in my bibliography, and I may only have 3 pages of writing, but as long as the paper is focused it will be well received.
Now, in some non-fiction books, you pick them up and there are hundreds of footnotes. For the most part, these are references and not explanations of text. Frequently, they expand the story - "This marriage later caused the War of the Roses (see also Dexter, 1989)." If the story doesn't cover the War of the Roses, it is appropriate. Unfortunately, fiction writers tend to use footnotes to explain the story further: "Sarah has blonde hair and blue eyes and is a very pretty girl." Again, this can be used well, but it is not recommended. Explaining the story (in footnote or text) is NOT necessarily expanding it.
Yes, all rules in literature are made to be broken. However, if you do not know them and understand how to use them and when to break them, it is best to follow them.