•• Here the speaker compares a book (in this case, a book of poetry) to a high-spirited, light-footed horse ("courser" is an old-timey word for a knight's horse). And guess what? She says that reading is even better than taking a ride on this great horse.
• Line 4 refers to poetry as "prancing," which cleverly plays upon the terms commonly used to talk about poetic meter, like "metrical foot" – for more on this concept, check out the "Form and Meter" section.