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

In ,
Calpurnia takes the children to her church, and Lee gives the audience a signficant view of
Cal's culture and background. andare 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).