Letters turn talking to yourself and to someone else into the same thing. The recipient can’t interrupt, and can’t answer back, at least not yet. Self-obsession is almost a virtue in letters since correspondents who won’t talk about themselves are boring. But letters also make for unreliable autobiography because they’re written out of an understanding not just of what the sender wants to say but also what the recipient needs to hear – and every recipient is different. This is why reading letters not addressed to you is taboo: you invade the privacy of two parties.