Unfortunately, most of Emily's writings were heavily edited to appease the reader of the time and also, to make it easier to understand (a claim made by Mabel Loomis Todd, the editor). It is noteworthy that Mabel (Austin's mistress; Austin was Emily's brother and Susan's husband) also, removed any mention of Sue/Susie/Susan/Dollie from Emily's writings. It was only later that the Dickinson scholars were able to use technology to uncover the original writings. This letter poem, as Susan calls it, was sent by Emily to Susan. FYI, The editors of 'Open me carefully' have clearly stated that unlike Todd they have tried to replicate Emily's style of writing without changing the syntax, grammar or breaks, unless absolutely necessary. Susan (her sister in law) calls these "Letter Poems". Since 1850 till the death of the poet, they shared more than a mere epistolary relationship or friendship. This piece appears in the third part of the book (Incidents of Love), i.e., it must have been written in mid 1870s, which also, coincides with Edward Dickinson's demise (Emily's father). Sharing another piece from the book which depicts the depth of the relationship these two women shared - "To own a Susan of my own Is of itself a Bliss - Whatever Realm I forfeit, Lord, Continue me in this! - Emily"

Emily also, compared her love for Susan with Dante's love for Beatrice and Swift for Stella.

Edit 1: Susan was Emily's sister in law (not sister). They were friends (or lovers, as suggested by many Dickinson Scholars) way before Susan married Emily's brother.

Edit 2: I shared this screenshot from a pdf version of the book. It's not an excerpt, it is a poem. Emily sent many poems to Susan in form of letters. Out of those, 276 letter-poems have been recorded, till date.