The Silver Chair is the penultimate novel in the series, as I read them in chronological order.
Narnia's benevolent deity Aslan has already told all the Pevensie children that they will not return; but Eustace Scrubb, who was swept along on their last adventure is the protagonist of this story, along with his classmate Jill Pole. The two of them are called to Narnia by Aslan to rescue King Caspian's son, who went missing ten years earlier.
As with so many of Lewis's other works, this one is filled with Biblical allegories and Christian messages. The world below ground ruled by an evil sorceress where the climactic battle takes place suggests Purgatory, if not Hell. It is clear that Lewis blames the lack of morals among Eustace and Jill's school on the school's rejection of God and religion. The school is so secular that neither child even knows the names of Adam and Eve.
Whether you accept Lewis's faith or not, the story is another good one. It is filled with wonderful characters, including the pessimistic, but charming Puddleglum; and with action and adventures and magical creatures.
I am looking forward to wrapping up the series tomorrow.