Sources: CBev3
- Samuel L. Clemens to John Brown, 22 and 25 September 1873 · London, England · UCCL03362
- Samuel L. Clemens to John Brown, 27 April 1874 · Elmira, N.Y · UCCL01080
- Samuel L. Clemens to Hjalmar H. Boyesen, 6 November 1879 · Hartford, Conn · UCCL01707
- Samuel L. Clemens to John Brown, 14 and 15 August 1880 · Elmira, N.Y · UCCL02814
- SLC to Albert Hamann, 9 August 1893 · Krankenheil, Germany · UCCL04433
1