The use of the marketing phrase, underground bestseller further illustrates the independent-from-sales, self-defining aspect of the term. For example, publisher HarperCollins suggested the bestseller potential of Divine Secrets of the Ya-Ya Sisterhood: A Novel by announcing "...four years after her award-winning, underground bestseller, Little Altars Everywhere ..." in the promotion. The book went on to achieve bestseller status in the 1990s. In reviews of the 2002 film of the same name, the novel's bestseller status was cited routinely, as in "compelling adaptation of Rebecca Wells' bestseller". 
Thanks for a terrific article explaining a complex and inexact process.
I spent nearly 30 years in librarianship and I was always troubled by ‘HOW’ titles and authors made that list. But finding a book which was ON that list … was a big plus for library users.
People are more predisposed to ‘like’ or enjoy something if they already know many others do also.
That said, I was briefly a ‘bestseller’ too … but it has to be qualified.
My brother and I co-authored a book in 1988 which was published by Libraries Unlimited, at that time one of the top three publishers of books for librarians. That book sold out the first and second printing and was in the third printing before it was ‘remaindered.’ In the . catalog for the next two years (’89 & ’90) our book was listed as a “bestseller”. And it was … for that company.
However, it was the good reviews which sold all those copies … not the designation in the . catalog.
A beautiful, stunningly ambitious novel about a blind French girl and a German boy whose paths collide in occupied France as both try...
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Author : Zebian,Najwa Mind Platter is a compilation of reflections on life through the eyes of an educator, student and human who experienced most of life in silence. It is written in the words of a person who came from Lebanon to Canada...
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Milk and Honey
Author : Rupi Kaur Milk and Honey is a collection of poetry and prose about survival. About the experience of violence, abuse, love, loss, and femininity.