Helena Petrovna Blavatsky conducted an extensive correspondence throughout her life. She was perfectly proficient at writing in Russian, in French, and in English.
Letters to A. P. Sinnett
A. Trevor Barker compiled The Letters of H. P. Blavatsky to A. P. Sinnett. The original letters are now in the British Library. The text has been made available online by the United Lodge of Theosophists - Phoenix, Arizona and at Theosophical University Press Online.
The Judge letters
William Quan Judge made what is probably the first compilation of HPB's correspondence in The Authentic Letters of H.P.B., As Edited by One Of the Main Founders of the Theosophical Movement, available online at Esoteric-Philosophy.com.
Boris de Zirkoff, followed by Dr. John Cooper, and then Dr. John Algeo, worked diligently to collect all the letters written by Madame Blavatsky, and to organize them in chronological order. Letters from 1861-1879 have been published in a first volume by Dr. Algeo in 2003, The Letters of H. P. Blavatsky.
A great deal of additional work is yet to be done in what promises to be a long series of books of HPB's letters. Several years after Dr. Algeo's retirement, American Theosophist Jon Knebel took up the task of completing the series.
Charles A. Blech collected letters and telegrams written to the early French members of the Theosophical Society, and published them in Contribution á L’Histoire de la Société Théosophique en France (Paris: Éditions Adyar, 1933), which is available in digital form from the Olcott Library. Letters from Helena Petrovna Blavatsky are provided in transcripts, and a reproduction of a letter from March 24, 1890 is included. She wrote directly to Commandant D. A. Courmes, M. Biliere, and Madame Lemaître, and addressed others such as M. Tremeschini via letters to the editor of journals. The date range was 1880-1890.
- "Letters of H.P. Blavatsky Online" at Blavatsky Archives has correspondence with Monsieur Billiere, Mrs. Hollis-Billings, Henry S. Olcott, Dr. Franz Hartmann, Dr. Wilhelm Hübbe-Schleiden, Dr. Elliot Coues, her family, William Quan Judge, and others.
- H. P. Blavatsky to the American Conventions, 1888-1891, available at Blavatsky Net.
- Some Unpublished Letters of H. P. Blavatsky, compiled by E. R. Corson is available at Theosophical University Press Online. There are altogether 17 letters. The correspondents are Professor Hiram Corson in 1875-1876; Mrs. Corson in 1876 and 1878 (written in French); and Alexander N. Aksakof (introducing Madame Corson, in French, 1878). This document is available at in two parts at Theosophical University Press Online - part 1 and Theosophical University Press Online - part 2.
- "Extracts from Private Letters", available at Esoteric-Philosophy.com. This text is reproduced from a three-volume collection, Theosophical Articles, by H. P. Blavatsky, Los Angeles: Theosophy Company, 1981. See volume I, pp. 115-123.
- John Algeo, The Letters of H. P. Blavatsky, Wheaton, IL: Quest Books, 2003.