The average length of a non-fiction book is 50,000 words, with some books falling at around 35-40,000 words, and others running as long as 70,000. This means, if you're writing a non-fiction book, your book needs to be about 50,000 words?--give or take a few thousand words?--right?

Not necessarily, according to a new study by Book in a Box of 272 non-fiction books that hit The New York Times Best Sellers list between 2011 and 2017. In 2011, the average length of a best-selling non-fiction book was 467 pages. By 2017, that number had plunged 42 percent to 273 pages.

The shortest book to hit the number one spot on The New York Times Best Sellers list? Harry Frankfurt's 80-page On Bullshit.

The longest? Robert Caro's 1232-page tome, Master of the Senate.

According to Book in a Box, over 64 percent of the number one best-sellers since the list began fell into the 200 to 400-page range. In recent years, however, this spread has narrowed, with over 50 percent of number one best-sellers falling between 250 and 350 pages.

It's tempting to attribute the ever-shrinking length of best-selling books to the pervasiveness of smartphones and shriveling attention spans. But some of the best-selling books of all time were published well before the iPhone ever appeared, and well before ADHD was recognized as a mental disorder by the American Psychiatric Association.

Strunk and White's classic guide to writing well, The Elements of Style, at a mere 105 pages, has sold well over 10 million copies since it was first published in 1959. Who Moved My Cheese?, a parable by Spencer Johnson about how people resist change at work, clocks in at just 94 pages. Number of copies sold since it was first published in 1998? 28 million.

While the Book in a Box study focuses on page number, it's worth noting that page count is becoming less of a relevant indicator of actual book length than absolute word count. That's because on a Kindle?--or a Kindle app on a smartphone?--page count is just another variable that changes once a reader increases or decreases the font size.

Nonetheless, their analysis highlights a trend that is likely to continue, and one with substantial implications for both readers and writers of books.

For in today's information-saturated world, in which thousands of new books, articles, and blog posts are published daily, sorting through this mountain of content and discovering what's relevant--and what's not?--?is made that much easier when content is shorter and more quickly digestible.

So if you're a writer looking to boost sales of your new book?--or if you're trying to place it on the highly coveted New York Times Best Sellers list?--make sure you track your word count closely.