It can be found on Craig Boone's body if he dies. Everyone that enters the NCR army is required to write a letter to be passed on to their loved ones if they die in action.  Boone had written this to his wife Carla, apparently during her pregnancy. Boone kept his note and carries it around with him, possibly as one more way to carry on her memory.
If you’re reading this, then you know. Sorry. Wanted to make it back home to you.
The pension won’t be much but it should help you and the baby get by. Want you to remarry when you meet the right person. Don’t want you to have to be on your own.
Not sure the right way to say how I feel about you. Think you know already, though. Always seemed like you knew what I meant, maybe better than I did. Wish I was there with you now.
There are things I couldn’t tell you. Tried. Whatever you learn over time about my service in the NCR, hope you can forgive me.
Lastly, know you were against it, but if it’s a girl, want her to be named after her mother. Know it’s playing dirty to win the argument this way, but too bad. It’s worth it.