What source does Douglass rely on to learn how to read and write?

His mistress, Mrs. Auld, first begins teaching Douglass the alphabet before her husband prohibits her from doing this. His severe opposition told Douglass how important reading must be:  “What he most dreaded, that I most desired.”  He thanked both the master and his wife for enabling his interest in reading: “In learning to read, I owe almost as much to the bitter opposition of my master, as to the kindly aid of my mistress. I acknowledge the benefit of both. After he learned from “the little white boys” how to read, he would always find a book and take it with him where ever he went.  Soon he started to read The Columbian Orator, which taught him about emancipation and changed his life even more. As for writing, Douglass recounts: "The idea as to how I might learn to write was suggested to me by being in Durgin and Bailey's ship-yard, and frequently seeing the ship carpenters, after hewing, and getting a piece of timber ready for use, write on the timber the name of that part of the ship for which it was intended."  He again practices whenever he can, painfully learning how to make letters, and then letters into words.