In "To Kill a Mockingbird", how did Calpurnia teach her eldest son, Zeebo, to read?

In Chapter 12, Calpurnia takes the children to her church, and Lee gives the audience a signficant view of Cal's culture and background.  Jem and Scout are in awe after their experience and ask Calpurnia about Zeebo's ability to read (since only 4 people in the church can read).  Calpurnia tells them,

"I made [Zeebo] get a page of the Bible every day, and there was a book Miss Buford taught em out of" (Lee 125).

The other book is Blackstone's Commentaries, a book about law (not too ironic since Calpurnia got it from the Finches).