The rankings for Amazon are calculated roughly every hour from data on book sales, but the factors considered by the e-commerce giant's algorithms remain largely a mystery.
"It is like the most closely guarded secret in the universe," Ellen Geiger, vice president and senior agent at Frances Goldin Literary Agency, tells CNBC Make It.
While it isn't clear exactly how many copies of a book must be sold to break into a slot on the list, a 2013 study of Amazon's best-sellers list for a two-week period by Publishers Weekly found that "a title in Amazon's top five averages 1,094 print copies sold across all channels, including other retailers, on a typical day."
Since Amazon updates its rankings so frequently, "really what it measures is velocity of sales ... over a short period of time rather than an absolute number of sales over a longer period," says Caroline Eisenmann, an associate agent at Frances Goldin. "What it reflects is how a book is selling relative to the other books on Amazon during any one time."
Still, that No. 1 slot in books is certainly an indication of a buying spike.
"If a book has cracked the top 100 and and particularly the top 20 and stays there — the duration is a huge part of it, if it stays there for a few days — that is generally a very strong sign that it is going to be a New York Times Best Seller and that it is going to start showing up on other lists," Eisenmann says. "To hit that top slot is huge. It means that a book is selling hugely during the period it is there."
Currently, the holder of the highest-selling nonfiction release is Hillary Clinton's "What Happened," which sold 300,000 copies in its first week, according to publisher Simon & Schuster, as reported by Fortune.