Like a mean teacher tripping up a keen-to-impress pupil, boffins have come up with a way of calling us on books we don't get through. Named after the author of that noted 1980s ornament, A Brief History of Time, the Hawking Index leafs through e-books and asks where readers wield the e-highlighter pen. The choice nuggets of Thomas Piketty's top-selling political economy 700-pager are clustered in pages one through 26; "readers" of Hillary Clinton's new shelf-stopper are making Hard Choices to stop at an even earlier stage. But what flagellant ethic could demand they do anything else? While covers continually wink from new books, lost time is not found again. Shrewd non-fiction editors pull highlights to the front, and writers who might once have despaired at half-digested books will today be more concerned by authorial incomes plunging to an average of £11,000. Readers, whether they finish or not, all help with that.