Writers whose books are marketed by conventional publishers have the benefit (or curse) of many pairs of eyes and many years of experience to influence and guide the development of a book before it goes to market.
Self-publishers often rely solely on their own judgement, but that's probably not a good idea.
Later today I will be uploading what I hope will be the final version of a book that I thought I had finished a few months ago (and it has been selling on Amazon.com). But the version that will be sold next week will be better than the "first edition."
I didn't do any formal test-marketing; but in talking to early readers, and noting the reactions (smiles, laughs, frowns, quizzical looks) of people as I passed a copy around the table at a few holiday parties, I realized that the cover could be improved.
Some people just didn't "get" what I wanted them to.
For example, I had a funny photo of a woman on the back cover and lots of people asked who she was. It was a stock photo that represented a character in the book, but it was difficult and awkward to explain her.
I realized that she was simply not needed, was an unnecessary complication, and that the space she was using could be better used for other things.
Additionally, my wife hated the photo and had been begging/nagging me to remove it. I seldom surrender to nagging, and now Marilyn got her wish -- but it was because I did what I wanted to, not because of the pressure.