A Western Union telegram from the 1940s. An “I Love You” scribbled in red pen on a book of matches from the late ’90s. A postcard, Post-it note, fraying napkin. An email, text message or string of emoticon. A million IMs. These are the shapes and forms, spanning nearly a century, of the myriad testimonials of the heart and head in Other People’s Love Letters: 150 Letters You Were Never Meant to See, edited by Bill Shapiro.