The book-publishing market has an element of mystery to it, and not only in the category of books called mysteries.
Many things are not as scientific as you might think.
Prominent book-bestseller lists are based on data from a sampling of booksellers, rather than comprehensive information outputs from all channels.
Industry-status reports from publishing trade associations use a similar sampling approach and not a comprehensive list of all data. Industry trends are estimates.
Over the last couple of decades, more and more publishers rely on Bookscan data as a source of information to make decisions. Now owned by the global company NPD, headquartered in Port Washington, New York, they provide weekly data updates from a broad spectrum of bookselling channels. Access is by paid subscription only, but many free reports and insights are distributed regularly. Click here.
Still, only the author and their publisher really know how many copies of a book sold. Author purchases are only tracked by the author and publisher. The same applies to sales to organizations and most international exports.
This is why every sales milestone (“100,000 Sold!”) for a book is really based on taking the author or publisher’s word for it. A book that sells 100,000 copies might actually be a combination of physical books, digital copies, export sales, audio downloads, nonretail sales, and author purchases.
Even the number of books published every year across the country and world are educated guesses. The closest estimates are usually gathered by counting ISBNs used from R.R. Bowker, the private company that is the official source of ISBNs for publishing in the US.
Compared to many other businesses or industries, some of the transparent data on books is a relatively new thing. A couple of decades ago, it was far more mysterious than it is today. But still, there are significant areas of publishing that remain hidden from the public. Hence, once again, only a publisher and author know how many copies a book actually sold.
A book published by any type of organization for their constituency will never find itself on a bestseller list but might sell substantial quantities.
Purchases by the author will never be reported to the media or the data-tracking services like Bookscan.
Years ago, a large church that had a bookstore began reporting its sales data to bestseller lists. Since most all of the sales of the pastor’s books were in one store, it was considered ineligible for reporting on national bestseller lists, even though the sales volume would have been sufficient to make a good showing.
National bestseller lists are national bestsellers, meaning sales must be broad and not in one or only a few locales, so the pastor’s books were excluded. It’s one of the many reasons bestseller lists have editors, ensuring the list portrays helpful information, reflecting trends in book buying.
Publishing used to be substantially more intuitive than it is today. These days, the data available to publishers and authors combines with intuition to make more informed and better decisions.
At least that’s the theory!