In the letters before each one there is a little paragraph that says who the letter is to and often what they were doing and in the letters themselves if you read th em carefully it tells what the boys were doing, where they were stationed and how t hey were doing.
For instance :
79 From a leter to Christopher Tolkien
( a reply to Christopher's comments in Kroonstad where he was stationed and on Johannesburg)
(Christopher had moved to a camp in Transvaal.
As you read through the letters JRR talks a great deal about what his boys are doing and his grief at Christopher flying a plane for the RAF which to him is a 'machine' and evil. He even talks about Rayner Unwin, the boy now grown up that originally read his first entered work and approved of it to his father the publisher.Really if you read enough letters you will learn enough anyway.
At one point he talks about his three sons finally being all at Trinity at the same time.