I came across this very famous letter written by Union soldier Major Sullvian Ballou to his wife, just 1 week before being killed in the war. It is a very touching letter.
Some of the lines from the letter that I find especially touching:
Forgive my many faults, and the many pains I have caused you. How thoughtless and foolish I have oftentimes been! How gladly would I wash out with my tears every little spot upon your happiness, and struggle with all the misfortune of this world, to shield you and my children from harm. But I cannot. I must watch you from the spirit land and hover near you, while you buffet the storms with your precious little freight, and wait with sad patience till we meet to part no more.
But, O Sarah! If the dead can come back to this earth and flit unseen around those they loved, I shall always be near you; in the garish day and in the darkest night — amidst your happiest scenes and gloomiest hours — always, always; and if there be a soft breeze upon your cheek, it shall be my breath; or the cool air fans your throbbing temple, it shall be my spirit passing by.
The full letter can be found here : My very dear Sarah.