Brother Men: The Correspondence of Edgar Rice Burroughs and Herbert T. Weston Book

Behold, unwrap the highest genius. Like Keats alone among poets, Dickinson's letters exhibit that genius. In fact, she compares winter to, " Keats's bird, 'who hops and hops in little journeys.'" Andrew Marvell's letters, for instance, are humdrum affairs mostly written in his public voice as parliamentary representative. ED writes with her poet's ear, "Friends are gems--infrequent" (II.352, 1859). Or, check this sentiment which would compound among moderns (even inaugural poets): "I have heard many notedly Bad readers, and a fine one would be almost a fairy surprise" (II.345, Jan '59).
Her verses punctuate her letters, letters which are often as epigrammatic as her verse; in her last year, "Fear makes us all martial." Apply that to the gun promoters today.

One two month visit to the UK for research, I would read a paragraph in Gilbert White every day, as a Naturalist's Bible. Decades earlier, I did the same with Dickinson's letters. One can open them at random, and find within a page or two something unprecedented and yet familiar, like this today.
After telling Loo (L Norcross) about the vegetable she sent, to be eaten with mustard, she observes:
"I enjoy much with a precious fly, during sister's absence, not one of your blue monsters, but a timid creature, that hops from pane to pane of her white house, so very cheerfully, and hums and thrums, a sort of speck piano."
Then she adds, almost as surprisingly, "Tell Vinnie I'll kill him the day she comes, for I sha'n't need him any more, and she don't mind flies" we'd say, she Does mind them (II.353).
There are revelations about her famous personal avoidance of others; from the friends/gems letter to Loo, she confides, "For you remember, dear, you are one of the ones from whom I do not run away! I keep an ottoman in my heart exclusively for you." And she reveals with profundity some of her avoidance: "My own words so burn and chill me that the temperature of other minds is too new an awe" (to Chickering, 1883).
Her epistolary attentions range far, including of course, her assessment of the form in which she writes," A letter always seemed to me like Immortality, for is it not the mind alone, without corporeal friend?" (1882).
And she even appears to have said, like the Reformation martyrs, her own last words, but in a letter: "Little Cousins, Called back."
Here she immortalizes a book title (by one Conway) she had read, so her last words are also a literary allusion.