By now, the poor sales of Hillary Clinton's new book "Hard Choices" are well-documented. (Relatively poor, we will add, given the complex topography of bookselling.)

But another metric came to our attention this weekend which allows us to loosely evaluate a more interesting bit of data: how much the book is being read.

Jordan Ellenberg, a professor of mathematics at the University of Wisconsin, Madison, outlined what he calls the "Hawking Index" in the Wall Street Journal over the weekend. The index is a way to estimate how far into books people actually get. It's named for Stephen Hawking, author of the dense "A Brief History of Time" which, swear to God, I've actually read. (In part.)

It works like this: Every time people highlight something in a book on their Kindles, Amazon records that data. Ellenberg takes the top five highlights listed at the site for any given book and correlates them to a page number. Comparing the average page number of those five highlights to the length of the book gives you a sense of how many people made it how far in. (He adds: "Disclaimer: This is not remotely scientific and is for entertainment purposes only!" Which, fine.) The summer's most-read book? Donna Tartt's "The Goldfinch." Least-read? Thomas Piketty's "Capital in the Twenty-First Century," for which the notations only get about 2.4 percent of the way in.

So, naturally, we decided to apply this methodology to "Hard Choices" and other recent or comparable political books. And we have our own ranking, which we now present in order from estimated-least- to estimated-most-read.

1. "Hard Choices," by Hillary Clinton. Hawking Index: 2.04 percent.
Well, there you have it. The deepest into Hard Choices the popular highlights get is page 33, a quote about smart power. Three of the five most-popular highlights occur within the first 10 pages. We will note the same caveat that Ellenberg applies to Piketty. "Hard Choices" is fairly new, and fairly long. Still, though, one would think more people had made it past page 33.

The most popular quote? "Do all the good you can, at all the times you can, to all the people you can, as long as ever you can." Which, like several of the top quotes from the authors listed below, isn't actually a quote from Hillary Clinton. Instead, it's a mantra from her family's Methodist faith.

2. "Promises to Keep," by Joe Biden. Hawking Index: 2.78 percent.

Oh, Joe. Joe, Joe, Joe. Did you know that Joe Biden has a book? Joe Biden has a book. And people haven't read very far into it.

The most popular snippet is a bit of Bidenesque feel-goodery: "The art of living is simply getting up after you’ve been knocked down." There you go! But it is hard to find the most popular highlights in the book, because there simply aren't enough to warrant placement on the main Kindle page.

3. "A Fighting Chance," by Elizabeth Warren. Hawking Index: 14.38 percent.

Clinton supporters have compared "Hard Choices" to "A Fighting Chance" in the past, perhaps because they recognize it compares well on sales. And Warren's book is also the third least-deeply read -- although readers are still making it a bit further in than either of the two Democrats most likely to run in 2016, apparently. The second-most highlighted quote in Warren's book is the one that goes deepest in, perhaps given the punchiness with which it begins: That the economic crisis "didn’t have to happen."

4. "My Life," by Bill Clinton. Hawking Index: 15.28 percent.

The other Clinton's (extremely long) memoir apparently hasn't been read as much on the Kindle, as befits its age. While the most-highlighted passage in "Hard Choices" has been highlighted 223 times (as of writing), the most-highlighted in "My Life" has only been selected 65 times. That passage? "I learned that what seems funny to the strong can be cruel and humiliating to the weak."

(Note: Biden's most highlighted passage has been highlighted nine times.)

5. "Living History," by Hillary Clinton. Hawking Index: 17.88 percent.

Clinton's first autobiography was, by all accounts, more of a crowd-pleaser. Unlike "Hard Choices," it focused broadly on her life and time in the White House. And people appear to have read it more.

As with "Hard Choices," the most popular quote isn't from Clinton. It's from Eleanor Roosevelt. "A woman is like a teabag. You never know how strong she is until she’s in hot water." The third-most-popular highlight is a good one, deriving from Clinton's Methodist faith: "Do all the good you can, by all the means you can, in all the ways you can, in all the places you can, at all the times you can, to all the people you can, as long as ever you can."

Sound familiar?

6. "Dreams from My Father," by Barack Obama. Hawking Index: 17.94 percent.

Doing slightly better than his 2008 nemesis, the most-highlighted quotes from Obama's autobiography appear a little deeper into the work, on average. Most of the popular passages deal with race, but the most popular is about life's challenges -- something everyone eats up. He refers to a friend from college: "You might be locked into a world not of your own making, her eyes said, but you still have a claim on how it is shaped. You still have responsibilities."

7. "Duty," by Robert Gates. Hawking Index: 24.55 percent.

Shortly after "Hard Choices" was released, Clinton supporters told Politico to compare sales to Gates's and Warren's books. That first one was a mistake. Gates's book has handily outsold "Hard Choices" (so far), and according to this ("not remotely scientific and for entertainment purposes only") metric, is being out-read, too.

On the plus side for Clinton, none of the most-highlighted passages in "Duty" are from his excoriation of Clinton's Iraq vote. But that was on page 376, and most people apparently only got as deep as page 300.