amanuensis -- from the Latin (servus) a manu, a slave with secretarial duties: meaning one employed to write from dictation or to copy manuscript.
This word often shows up as a euphemism of sorts for a ghost writer -- My Turn at Bat by Ted Williams, as told to John Underwood; Iacocca by Lee Iacocca with Ralph Novak; or Learning to Sing by Clay Aiken and Allison Glock.
The word appears in texts about the Bible in the Middle Ages. For example, God's Secretaries: The Making of the King James Bible refers to the men who in 1605 refined, corrected and improved previous translations of the Bible as "Amanuenses of the words of God." (Note the plural.)
By using this word, a skilled writer is usually implying with a wink that there was a lot more than just dictation and pure secretarial work being done ---as there usually is, even in pure secretarial work!