The average New York Times Bestseller for Non-Fiction has decreased in page length from 467 pages in 2011 down to 273 pages in 2017. This is a huge trend in the publishing industry that many people are not aware of, but for authors this type of data is invaluable.
In the last 3 years, more than half of non-fiction books that appear on the NY Times Bestseller list are between 250-350. Each year since 2011 the average page count has fallen over 42%, which is quite significant.
There is a correlation between the rise of smartphones and tablets and the average length of a book decreasing in size. Maybe this is because people can only stare at an LCD screen for so long and people can only read for so long before their eyes get tired.