Many of you who are researching self-publishing will have already written a book or two and be ready to publish, and that is awesome. However, if you have not yet started writing your book, you might be at an advantage.
I say this because you are taking the time to find out BEFOREHAND what type of book sells, and you smart enough to learn from the mistakes of others before you begin the journey for yourself.
So...what kind of book do you want to write?
Because your book MUST be a on subject of which you are passionate. If you are not passionate about your writing, it will most certainly show in your finished product, (assuming, of course, that you actually finish the book... and that is a long shot since writing a book is difficult enough without having a love of your subject).
Secondly, do a little research on your subject. If your subject is too broad, your book is at risk of getting lost in the vast sea of ebooks available on the internet. It helps to look for a niche in your subject. For instance, rather than writing a generic romance novel, write a novel containing polygamy, interracial couples, or even BDSM like E. L. James did in Fifty Shades of Gray. The goal is to make your book stand out from the rest.
Next, you want to make sure your book contains proper grammar, spelling, and punctuation. I cannot tell you how irritating it is to see glaring grammar, spelling, or punctuation mistakes in an ebook by an Indie Writer since it not only reflects poorly on the author, but the Indie Writer community as a whole. So please... do us all a favor and get your book proofread if there's any question of poor basic form.
You will need a good copy of Microsoft Word to get started. In the beginning, I always set my books up with one inch margins all around, double-spaced, with a Courier New, size 12 font. I do this because I want to know the traditional word count so that I will be able to tell approximately how many pages my book would contain in printed book form. A book with the above settings would contain 250 words per page, which is standard for traditionally printed books. (In other words, one page of type equals one printed page.) Later, it is fairly easy to change the font, margins, and spacing to format the books for each of the four ebook publishers I use.
Before you choose a title for your book, be sure to consider keywords that are going to help you sell your book and try to include them in the title in order to boost exposure and sales.
Most importantly, relax and enjoy yourself! Write for the sheer joy of writing, then if your book does sell, it is a bonus but not the main prize.