How did Pullman change the plot of Paradise Lost for The Subtle Knife? Why might he have kept the elements he did? To what end did he change others?

Pullman draws from many literary sources for his work. Pick one, such as The Chronicles of Narnia series, and compare. In what way are the books parallel in terms of plot, themes, tone, and characterization? How are they different?

What do the Specters symbolize?