Summary from Goodreads:
If I've learned anything from comic books, it's that everybody has one thing 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 Rider underneath. Apart from my comic books, Dad's letters are the only things I read more than once. I know which ones to read when I'm down and need a pick-me-up. I know which ones will make me feel like I can conquer the world. I also know exactly where to go when I forget Mom's birthday. No matter what, each letter always says exactly what I need to hear. But what I want to hear the most is that my dad is coming home.