Sources: NNPM
- Samuel L. Clemens to Shirley Brooks, 23 September 1873 · London, England · UCCL00963
- Samuel L. Clemens to the Editor of the Hartford Courant, 13 April 1874 · Hartford, Conn · UCCL01077
- SLC to James R. Osgood, 5 January 1883 · Hartford, Conn · UCCL02320
- SLC to Charles A. Dana, 11 July 1883, Elmira, N.Y · UCCL13323
1