What do Jem and Scout learn about the hymnals in Calpurnia's church?

In Chapter 12, Atticus leaves for Birmingham and Calpurnia takes Jem and Scout to First Purchase African M. E. Church for Sunday service. When the children arrive, they notice several differences between Calpurnia's church and the church they regularly attend. Scout mentions that First Purchase is unpainted, has pine benches instead of pews, and there is no organ, piano, church program, or hymn-books inside the church. After Reverend Sykes' opening remarks and a collection that is taken up for Helen Robinson, Cal's son, Zeebo, leads the church in several hymns. Scout is confused as to how the congregation is supposed to sing the hymns without any hymnals. After each line that Zeebo would sing, the congregation would pause, then repeat the line when Zeebo ended. Later on in the chapter, Jem asks Calpurnia, "but why do you all sing hymns that way?" (Lee 165) Calpurnia explains to Jem that is called linin'. Lining is a technique used where the leader sings the first line of a song, and the congregation carols the same line back. Calpurnia tells Scout that it wouldn't make much sense to have hymnals in their church because the majority of the congregation cannot read. She goes onto to explain how Miss Burford taught her how to read. Then, Calpurnia tells the children that she taught Zeebo how to read using only the Bible and Blackstone's Commentaries.