From the book:

Nowhere is the human being more truly revealed than in his letters. Notin literary letters - prepared with care, and the thought of possible publication - but in those letters wrought out of the press of circumstances, and with no idea of print in mind. A collection of such documents, written by one whose life has become of interest to mankind at large, has a value quite aside from literature, in that it reflects in some degree at least the soul of the writer. The letters of Mark Twain are peculiarly of the revealing sort. He was a man of few restraints and of no affectations. In his correspondence, as in his talk, he spoke what was in his mind, untrammeled by literary conventions. Necessarily such a collection does not constitute a detailed life story, but is supplementary to it. An extended biography of Mark Twain has already been published. His letters are here gathered for those who wish to pursue the subject somewhat more exhaus-tively from the strictly personal side. Selections from this correspondence were used in the biography mentioned. Most of these are here reprinted in the belief that an owner of the "Letters" will wish the collection to be reasonably complete.