When a well-connected author publishes a new book, they make the rounds of various shows. Such authors also enjoy mentions in the media, as befitting their fame and connections. For example, favored authors will often be interviewed and reviewed on NPR. Authors who have their own shows plug their own books. Elite authors are also supported by other cultural elites. These are the people, such as Oprah, who tell the rest of us what is good.
There is considerable advantage to being blessed by the curators of culture. First, there is the boon of exposure. Being a gamer, I will present this in gaming terms. One way to look at this is that a certain percentage of people will buy a book if they hear about it. Alternatively, this can be thought of in terms of a percent chance that a person who hears of the book will buy it. So, for example, a book with a 5% purchase rating would be bought by 5% of those who hear about it. Or each person who hears about it has a 5% chance of buying it. While this is an abstract, game style simplification, it shows that the more people who hear about a book, the more copies of a book will be sold. This is true even for books that are not good. This is the principle that spam works on: if enough people hear about something money can still be made even if the response or purchase rate is low. Obviously when an author gets on a significant show to talk about her book, her sales will increase. Likewise for other forms of exposure for the author and the book. Equally obvious is that access to the curators of culture is limited and carefully controlled—an author must be properly connected to make it into the brightest circle of media light. This connection might even be luck: the book just happens to catch the attention of the right person, and the author is invited, perhaps briefly, into the circle.
Second, there is the gift of endorsement. If a book is endorsed or praised by the right people, this can boost to sales over and above the boon granted by exposure. While endorsement does provide exposure, exposure does not always entail endorsement. After all, the curators of culture do sometimes speak of books they dislike. While condemnation of a work might impair its sales, the exposure can increase sales. And being condemned by the right sort of people can boost sales. In the case of ideological works, for example, being condemned by an ideological foe could boost sales among ideological friends.
As discussed in an early essay on luck, the quality of a work has little connection to its success. Luck is a major factor. Exposure and endorsement add to this, and either might be acquired by luck). In an ideal aesthetic world, works would receive exposure and endorsement proportionate to their merit. In the actual world, there is often little correlation. The best books need not be the most exposed or most endorsed. Mediocre (or worse) books might garner great attention and receive unwarranted praise from the curators of culture.
This is not to say that merit never achieves success, just that merit seems a small factor in successful sales. Sometimes, just sometimes, a meritorious work does achieve success against long odds because of its merit—but this is notable in its rarity.