I recently read a Guardian article on the success of self-published e-book authors.
The Guardian article refers to such books as "hidden" bestsellers. Most e-books do not have ISBN's and, as such, are not captured in official book charts. They fly under the radar. But this does not mean that they are reaching smaller audiences.
Moreover, self-publishers operate under increasingly favorable financial terms. Books released on Amazon's self-publishing division, Kindle Direct, generate up to 70% royalty on sales, while traditional published books earn only about 7.5%.
The high costs associated with publishing and distribution in the traditional book market incentivize publishers to be risk-averse (in close similarity to Hollywood). These costs do not exist when self-publishing. Therefore, authors are free to take risks, experiment, and cater to the long tail of potential fans.
This model has parallels in the worlds of music and gaming. It's one of the positive upshots of the internet impacting creative industries.