The first time Bill Shapiro says he read a love letter addressed to someone other than himself was in the home of his ex-girlfriend, behind her back. He says the moment sparked a desire to see more of other people's love letters. Gayle talks with Bill about his book Other People's Love Letters: 150 Letters You Were Never Meant to See and about why the idea of other people's love letters is so alluring.

Some of the letters featured in the book are romantic, others are sad and some are downright crude. "Any of us who have been through a serious relationship knows there will be ups and downs," Bill says. Some of the letters belong to Bill's friends and others come from strangers. He says they range from flowery notes from the 1930s to a quick text message from 2006. "Part of the mission was to be very inclusive—all over the country, all different kinds of people, all different ages and different kinds of letters, e-mails, text messages," Bill says.

The letters capture the emotions felt in different stages of relationships from the "morning after" to the day of a breakup or divorce, Bill says. Gathering the 150 letters took about a year and Bill says the result of his research is a peek into a fascinating world that usually remains private. "You know so much about your friends and you talk about all sorts of different things, but you never show love letters and so that is why I thought this was such a great opportunity to show that part of us that is hidden," he says.