If there’s one thing I’ve learned from comic books, it’s that everybody has a weakness—something that can totally ruin their day without fail. For the wolfman it’s a silver bullet. For Superman it’s Kryptonite. For me it was a letter. With one letter, my dad was sent back to Afghanistan to fly Apache helicopters for the U.S. army. Now all I have are his letters. Ninety-one of them to be exact. I keep them in his old plastic lunchbox—the one with the cool black car on it that says Knight...