Have you ever wondered what those labels on book covers with "The Sunday Times bestseller" or "The New York Times bestseller" or similar ones mean? What is the connection between those newspapers and the books? Does this mean that these newspapers recommend those specific books that have this label on the cover? What is it based on?
Let's take the example of "The Sunday Times bestseller" label, like I have on one of my recently purchased books. Turns out that the Sunday Times publication has a bestseller list that they update weekly. This list and the ranking of books on it are based on the number of sales those books have made the previous week in UK, among other factors.
The Sunday Times seems to be using data from Nielsen BookScan, which is a sales tracking system that collects sales figures from a wide range of outlets, including both physical bookstores and online retailers. Booksellers across the UK and Ireland submit their sales figures to Nielsen BookScan within a specific timeframe each week, typically between Sunday evening and Monday morning. These figures must be submitted in a specific format to be counted:
A csv file that is analysed by a computer and the information fed in to create the results. Late entries are not accepted, and so authors and publishers are really at the mercy of booksellers to submit that data to be in with a chance of making the bestsellers’ list. - according to "How Do You Make A Sunday Times Bestseller?" on The Book Room
In conclusion, whenever you buy a book that has one of these labels on its cover it means that the book has been in the "bestseller list" of that specific publication mentioned on the cover for at least one week. Each publication focuses on specific markets, for example The Sunday Times on the UK market, while The New York Times bestseller list is using data from the US market.
Although they are called "bestseller lists", other factors might contribute to the ranking, sometimes the actual sales might play a less significant role in the decision of the publication to include a book on their list. The New York Times at least was forced to admit that the list is not based on sales data only and that in the end their list is an editorial product, not an objective compilation of information.