You list as many sub topics as you can think of, and then you categorise them into 10, one for each chapter.
Then, for each chapter, you write 625 words, which really isn't much. Then from that, you write a 125 word intro and a 125 word conclusion. And you do that for each chapter.
Now you have 625+125+125=875 words per chapter = 8750 words.
Then you write a 250 word intro to the book, and a 250 word conclusion to the book.
Now you have a 9,250 word ebook.
You put it on amazon and see what happens. I don't know what a reasonable price for a book of that length would be (does anyone know?)
If people are buying it, then you expand on what you wrote, making everything (chapters and their intros and conclusions, and intro and conclusion to the book) twice as long, giving you an 18,500 word ebook.
Then you replace the original ebook with the longer version, and increase the price.
Then if that goes well you double it again, expanding on what you've written, giving you a 37,000 word ebook.
Does that sound like a reasonable way to do it? I think it does because it gives you a chance to just get on with it without too much to think about at first, and it's manageable, and then you just elaborate more and more, but at least you can get your book out there asap.
What about some other ways to write an ebook quickly and efficiently?