Emily D. quoting Emily B.

The Huffington Post informs of a very interesting auction taking place in December of famous authors memorabilia. No Brontë letters as far as we know but there is one particular item that is, in a way, Brontë-related. An Emily Dickinson letter:

Profiles in History has announced the items of famous author memorabilia that will be included in their December 18 auction.
The auction will take place in Los Angeles, and include 300 of the most important manuscripts in their collection.
The entire collection has over 3,000 items, and is expected to be sold over the next two years.  (...)
Emily Dickinson letter
This letter was to Mrs. Samuel E. Mack, dated Autumn 1884. Dickinson expressed pleasure at Mack's recent visit, and quoted an Emily Brontë poem, "Last Lines." (aka No Coward Soul is Mine). Letters from Dickinson are extremely rare. This is expected to sell for $20,000-$30,000.

Via EmilyDickinson.it we can read the complete letter:

It was very dear to see Mrs Mack.
A friend is a solemnity and after the great intrusion of Death, each one that remains has a spectral pricelessness besides the mortal worth -
I hope you may live while we live, and then with loving selfishness consent that you should go -
Said that marvellous Emily Bronte

Though Earth and Man were gone
And Suns and Universes ceased to be
And thou wert left alone,
Every Existence would Exist in thee -

Tenderly,

Emily -

It is well known that No Coward Soul is Mine was read at Emily Dickinson's funeral by Thomas Wentworth Higginson.