Dear Madam —
Literary epistles are often used to turn a plot or deliver a revelation, such as the letter never delivered in Jane Eyre, in which her uncle expressed a desire to make her his heir:
Will you have the goodness to send me the address of my niece, Jane Eyre, and to tell me how she is? It is my intention to write shortly and desire her to come to me at Madeira. Providence has blessed my endeavours to secure a competency; and as I am unmarried and childless, I wish to adopt her during my life, and bequeath her at my death whatever I may have to leave.
Or consider the letter in Wide Sargasso Sea in which Daniel Cosway impugns Antoinette's character and lineage to her new husband (Jane Eyre's own Edward Rochester): "Dear Sir. I take up my pen after long meditation, but in the end the truth is better than a lie…. "
Such letters, because of the way they introduce substantial plot twists in short order, are hard to pull off and may smack of being devices, if not used judiciously.
Letters can be slow and subtle, too. Consider the epistolary frame opening of Mary Shelley's Frankenstein, where several letters from the arctic explorer Robert Walton to his sister set the stage for the story. From the opening sentence of the first letter, with is references to rejoicing, disaster, enterprise and evil forebodings, through the third letter, we hear nothing about a man named Victor Frankenstein nor his sad and wrathful daemon. Our guide, Walton, has not met or heard of them yet. We learn instead of Walton's voyage north, his loneliness, his ambition to locate the Pole. Only in the fourth letter, with its mention of a dog-sled driver "of gigantic stature," do the more obvious gyres of the plot begin to turn. But the first letters are crucial. We are given the exquisite arctic setting and we are being prepared to read a story of hubris, of ambition and noble intent gone devastatingly wrong. The letters also create a universalizing layer of distance between the supernatural side of the story and us, the readers. No, we may never have met a being like Frankenstein's creature, but we've all met men like Walton, and he comes off as such a good, reasonable man, there's no reason not to trust him, so how can we doubt the story he then unfolds?
Write a letter that does not overtly advance your plot but which does establish character, voice, setting, reliability or some other essential background for your story. Don’t forget to consider the relevance of the letter’s addressee as well as its author.