Classic double signature “S. L. Clemens” and “Mark Twain,” on a flyleaf, 6 x 9, autographed by Clemens in 1906 for binding into The Writings of Mark Twain, Definitive Edition, published by Gabriel Wells in 1922, opposite an attached page signed by Clemens’ biographer and literary executor, Albert Bigelow Paine (1861-1937) explaining these circumstances. Attached to the half-title page of volume one (not present) of the 37 volume Definitive Edition The Writings of Mark Twain and numbered 299 (of 1024) and includes the steel engraving portrait of Twain rendered (according to the attached tissue guard) “in 1868 at the time of writing Innocents Abroad.” Paine’s collaboration with Clemens originated in 1906, when after just completing a biography of cartoonist Thomas Nast, he approached the writer about authoring his biography. Paine became Clemens’ literary executor and published his biography in three volumes in 1912 followed, in 1917, by the publication of Twain’s selected letters. Additionally, Paine oversaw the printing of works unpublished during Clemens’ lifetime. In fine condition, with a touch of light soiling. LOA John Reznikoff/PSA/DNA and R&R COA.