What source does Douglass rely on to learn how to read and write as explained in Narrative of the Life of Frederick Douglass, an American Slave?

Mrs. Auld is a weaver by trade and has never had slaves "under her control
previously," so she treats Douglass with kindness initially, just as she would do for any
other human being. She realizes that he cannot read and first teaches him how to recognize
letters and then how to form short words. This basic instruction completely changes Douglass's
life, as it gives him the needed tools to begin to build a sense of order in the printed world
around him.

Mr. Auld's reaction solidifies the importance in continuing this
journey. When he tells his wife that she cannot give Douglass any further instruction because
Douglass "should know nothing but to obey his masterto do as he is told to do,"
Douglass realizes the power of literacy. His quest to further his literacy instruction becomes
his passionate desire, and he gets pretty creative in following up on that...