Is it true that 90% of self-published books fail due to poor editing? How crucial is professional editing for new authors?

No … basically no.

Probably around 95% or 98% of all self-published books fail to sell a significant number of copies for one or (more often) more than one of the following reasons:

A) They are terribly written.

I feel I should explain what I mean when I say “terribly written,” so here:

i) She did not want to use a tricycle to go home, and she did not want to offend Ibrahim as well. She reluctantly stylishly sat at the back seat of the tricycle grinning blushingly.

ii) The day now broke and I felt the warmth of the sun on my face, it was Sunday, Chillicothe is where I wanted to go but knew that I had to prepare for the first day of the new school year. The first year of high school! I Already had went shopping so I just grabbed one of the five outfits I had bought and lay it on top of my dresser.

Lack of editing is not the problem when someone is writing sentences like the above. The problem is that the author has not yet learned to write adequately.

It doesn’t matter that someone with better competence in English could pick up the sentences above and rewrite them from front to back to remove the errors and make them readable. What does matter is that sentences this bad strongly correlate with unworkably terrible plots, inadequate scene development, unbelievable characters, and so forth — all the other problems that make a novel unreadably bad. In both cases above, the description provided for the novel constituted a complete plot summary. In both cases, the plot was not workable.

Here is a paragraph from the description of a third actual self-published book that illustrates the sort of problems seen in the plotting of really bad novels:

iii) Ash gray smoke fills the air with loud explosions of bombs everywhere, and lets just say it’s not what Aleksander DePru has imagined his sixteen year old life to be like. Aside from the world in a war, Alek has still been haunted by the way his father has died and left him to lead the army for America.

I trust no one thinks this sounds like something where an editor could put in the correct apostrophes and fix up the verb tenses and there you go, all done. It’s possible for a skilled author to justify a plot where command of a national army is handed to a sixteen-year-old kid. Does anybody think this author will have succeeded in justifying that particular detail in a believable way?

A skilled author can get away with remarkably silly plot elements. But an unskilled author can’t pull that off. That’s part of “not yet having learned to write adequately.”

B) They are terribly presented.

If the covers are dreadful and the description missing or awful, no one will buy the book. Editing still doesn’t matter at this point.

C) They are fake books.

They are nine pages long, or even fifty pages long — too short to be a book. Or they were generated wholly or in substantial part. Or both. Editing doesn’t matter for anything in this category.

D) They are unproofed or badly proofed.

Proofreading isn’t exactly the same thing as editing; or rather, it’s easier to discuss books and quality if one separates developmental editing from proofreading.

Inadequately proofread books that are filled to the brim with thousands of typos are unreadably bad. That will certainly cause a book to fail.

E) They are terribly formatted.

If the formatting is just really weird and un-book-like, that will turn a lot of readers off. If a book is supposed to appeal to readers, then it needs to be formatted like a normal book.

I’m not sure why this is such a common problem, as literally every traditionally published book is formatted properly. All anybody ever needs to do is open any ebook that was traditionally published, or any ebook in the top-selling 100 for the genre, and look at the formatting. That’s how the interior of a book should be formatted. It’s not rocket science to make an ebook look like that, though it may take some fiddling.

F) The book is published without any promotion at all. It is not promoted at all after being published. There’s nothing wrong with it. It might even be great. It fails anyway, because readers can’t buy a book unless they know the book is there.

The above are reasons self-published books fail completely. Here is one problem that does not cause books to fail.

G) They are unedited.

I mean they did not have a developmental edit and the author wasn’t capable of doing this on their own — some are — and therefore there are big, obvious problems with the plotting.

This is a shame, because the book could be a lot better. But readers are surprisingly forgiving of big problems if the book appeals to them. I’m a lot more aware of pacing, plotting, and story structure than most readers, but even so, I’m pretty forgiving myself, as long as the book is enormously appealing for some reason. I can point directly to big problems with plotting, but explain why I like the book anyway.