Analyzing Rhetorical Strategies in Frederick Douglass's Letter to Harriet Tubman

Rochester, August 29, 1868 
Dear Harriet:
 I am glad to know that the story of your eventful life has been written by a kind lady, and that the same is soon to be published. You ask for what you do not need when you call upon me for a word of commendation.  I need such words from you far more than you can need them from me, especially where your superior labors and devotion to the cause of the lately enslaved of our land are known as I know them. The difference between us is very marked.   Most that I have done and suffered in the service of our cause has been in public, and I have received much encouragement at every step of the way.  You, on the other hand, have labored in a private way. I have wrought   in the day – you in the night.   I have had the applause of the crowd and the satisfaction that comes of being approved by the multitude, while the most that you have done has been witnessed by a few trembling, scarred, and foot-sore bondmen   and women, whom you have led out of the house of bondage, and whose heartfelt, “God bless you,” has been your only reward. The midnight sky and the silent stars have been the witnesses of your devotion to freedom and of your heroism.  Excepting John Brown1 – of sacred memory – I know of no one who has willingly encountered more perils and hardships to serve our enslaved people than you have. 
Much that you have done would seem improbable to those who do not know you as I know you.  It is to me a great pleasure and a great privilege to bear testimony for your character and your works, and to say to those to whom you may come, that I regard you in every way truthful and trustworthy. 

Your friend, 
Frederick Douglass.