Sometime before the end of the month, when Republicans hold their convention in Tampa, Fla., Mitt Romney will announce his vice presidential running mate.

There's a good chance the finalists for that spot are wading through mountains of paperwork, and answering deeply personal questions about finances, past statements, friendships — and medical history.

A lot of that tedious process stems from something that happened 40 years ago this summer, when presidential candidate George McGovern decided to place Thomas Eagleton on the Democratic ticket. Joshua Glasser tells the story of that fateful decision in his new book, The Eighteen-Day Running Mate....