Faramir is a writing sort, but he’s not the type to waste words. Boromir, less of a scholar, less fond of writing, but more the sort to relate details and stories which may be less than relevant. Which I think has interesting implications on the correspondence between them.
so in my headcanon, Boromir’s the long-letter writer (his fellow soldiers are always like ARE YOU WRITING TO A GIRL WHO ARE YOU IN LOVE WITH and he just rolls his eyes because he’s writing to his kid brother) with the occasional short missives that are either military correspondence or completely irrelevant nonsense that he thinks will make Faramir laugh (and because they MIGHT be military correspondence, Faramir is obliged to open them as soon as he receives them. this leads to him making a lot of strange faces in front of his men.)
and Faramir is the one who can get across everything he wants to say in three sentences or so (like, short enough that Boromir has the whole thing memorized by the time he reads it twice) but sometimes he will sit down and write PAGES UPON PAGES, just to get out his thoughts, and when Boromir gets those he knows something is happening in Faramir’s head.
so when Faramir goes through Boromir’s room after his death, he finds all the letters in a drawer and so he brings out all of the old ones he’s kept and sits on his brother’s bed and reads them. he manages to keep from weeping until he comes across one of the short ones Boromir sent a few years ago, which contains a particularly stupid joke and came by the hand of a messenger who was under the impression that the letter was urgent. and he can hardly bear to think that the one who considered laughter so urgent is dead, and will never send him absurd letters again.