I've found that going to Amazon.com and putting in your subject will bring up a list of books that are based on your niche. You can check the table of contents to find specific topics to go into detail about. You can also check the information about the book, reviews, and recommended books for more drilled down topics in your niche.
Another thing that is helpful is to check the reviews of highly reviewed books. You'll find that the ones who didn't like the book or gave it 3 stars or lower often have specific content they wanted from the book. If they are true followers of the niche, they will go into detail about it and you can see what that target audience wants from that information.
Viewing forums, highly visited blogs, and Q&As are also helpful for this information.