I had several conversations last week about 'niches' and how to define them. There are many smart people on this forum who can help, but I wanted to suggest a book I ran across recently that discusses over 70 niches with commentary on each one.
Title: Microtrends: The small forces behind tomorrow's big changes by Mark J. Penn.
Not only does the book discuss niches (which could all be targeted by IMers) but it provides examples of niches. Many folks starting out have a hard time determining and finding appropriate niches. They can't convert the theory into specific examples.
I've suggested this book to many people, and they've all returned to me saying they appreciate reading about concrete examples.
I hope others in the forum can gain some insight from this book.