which is the best e-book you have wrote so far?

I have not written any Ebooks, and I don't think that this is necessarily a profitable way to make money either, although I am happy to listen to anyone who has proof otherwise. Most people publish Ebooks for vanity. No publishing house wants their book, and so they resort to publishing it themselves. This is fine, but it is not the same as having a publishing house like Random House or HarperCollins or Knopf or any of hundreds other legitimate publishers take on your book. When you self publish, you do all the work yourself, including looking for people to buy it. Lecture aside, if you are looking for hot topics, I would assume that most people who buy Ebooks are looking for titles that will help them make money. Otherwise, why not just go to the library or bookstore and find the topic you need? I am not trying to discourage you, just paint a realistic picture of the world of Ebooks.