Before his doomed 2008 presidential run, then-Sen. Joe Biden, following the unofficial candidate tradition, released a precampaign memoir, Promises to Keep: On Life and Politics. The book recounts his childhood and his staggering 39 years in politics up to that point (by the end of his vice presidency, it was 47). In it, he explains away the plagiarism scandal that effectively ended his 1988 presidential run by saying his words felt “absolutely authentic.” At the very least, he learned a lesson: ”You should not run for president because tactically you can win,” Biden writes. “The questions you have to ask are why you’re running for president and what will you do when you are president. You shouldn’t run until you know the answers to those questions.” Biden has run again for president, proving that a...