At several points in A Life To Kill (and I have to be careful with plot spoilers) I mention letters home from soldiers in Afghanistan. I had learned that many soldiers carried them in their breast pocket so that they would be easily found in the event of their deaths.
I researched these letters and found numerous examples. They were all simply written and full of love. These are probably some of the most truthful documents you will ever find. This link to the BBC website gives a little flavour of the sort of things I was looking at and which I hope I have managed to recreate in the book.