Emily Post (1872-1960) was an American author who wrote on etiquette. In 1922 her book Etiquette in Society, in Business, in Politics and at Home (frequently referenced as Etiquette) became a best seller, and updated versions continued to be popular for decades. Actually, it is still alive.
You can read the whole book here; it is a good tool in order to understand that time. But I'm sure the most interesting chapters for letter writers are XXVII (Notes and Shorter letters) and XXVIII (Longer letters). Maybe you will find some of her guidelines old-fashioned, but writing letters in the 21st Century... is not completely out of style? I found some interesting ideas:
The letter you write, whether you realize it or not, is always a mirror which reflects your appearance, taste and character. A "sloppy" letter with the writing all pouring into one corner of the page, badly worded, badly spelled, and with unmatched paper and envelope—even possibly a blot—proclaims the sort of person who would have unkempt hair, unclean linen and broken shoe laces; just as a neat, precise, evenly written note portrays a person of like characteristics.
A perfect letter has always the effect of being a light dipping off of the top of a spring. A poor letter suggests digging into the dried ink at the bottom of an ink-well.