I recently noticed something in a book I’ve had for five years: the bibliography section ends with a histogram of publication dates for references. I’ve used the book over the last few years, but maybe I haven’t needed to look at the bibliography before.
An up-to-date bibliography isn’t always necessary. One of the references I use the most is nearly 60 years old, and another book I pull down occasionally is 100 years old. But sometimes you do need a current reference, and a histogram of publication dates is a good way for the author to demonstrate that the references are up to date.
By the way, notice the phantom diagonal lines across the histogram? That’s a moiré.