Most people would go through thousands of rehashed articles from article directories which is not very productive. So here is what one should do- Subscribe to as many newsletters from various sites as possible in your niche.
For example- If you want to write an ebook on the weight loss niche then google "Weight loss newsletters" and you will have several big sites offering free newsletters.
The more sites you subscribe to the better it would be. I have observed that newsletters contain the best content and the most effective ideas which aren't really discussed on normal websites.
Therefore if you subscribe to 100 sites...You will have 100 emails in your inbox and 100 ideas to work on with extremely high quality content to do research on.
Also different emails will give you different ideas to formulate the layout of your ebook and naming the chapters.
If you want to speed up this process further you can search for various newsletter archives and you will find a lot of gold almost instantly. Let me know what you guys think of this.