If you have a nonfiction book in a very niche subject area, it may be difficult to prove to publishers that a general trade audience exists for the topic. (cooksplusbooks.com)
An exception is if you're writing on a very niche subject and there isn't much competition. (bookbuzzr.com)
The 99 cent price is critical to getting sales up and running from an unknown fiction writer, as opposed to nonfiction, where readers often have limited choices for a particular niche subject. (fonerbooks.com)