Niche Website's Goals
The primary goal (objective or mission) of a niche website SHOULD NOT be making money online. That's the wrong approach.
Helping your visitors with quality content that addresses whatever they were looking for when they visited your site OUGHT TO BE your niche site's primary mission.
To build something of value, worry about solving a problem or answering a question your visitors might have. That's the right approach.
DON'T FORGET: Making money on the Internet is TOTALLY different from earning money online!!!
Our "River Fishing" Example Again
If you wanted to build a niche site focusing in the “river fishing for trout books” niche, you'd need to create a site where you'd reviewed books (your own or from affiliate networks like Amazon) on that subject.
You ought to write quality review posts where you'd answer your visitors doubts and questions about those kind of books.
(BONUS TIP: Whenever you're looking for niche website ideas look for problems or questions you can solve or answer. That's the golden rule.)