Complex sentence: The second letter is from a college classmate of Pulver's who is also assigned to the "Livingston", which reveals that Roberts was killed in a kamikaze attack shortly after the first letter had been posted.
Split into simpler sentences: The second letter is from a college classmate of Pulver's who is also assigned to the "Livingston". The letter from Pulver's classmate reveals that Roberts had been killed in a Japanese kamikaze attack on the "Livingston" shortly after the first letter had been posted.