They deal with different aspects of the Christian life. The rallying phrase of the last book is farther up and farther in. CS Lewis, imho, doesn't reach the pinnacle of his writing in these books, but these allegorys paint a picture which helps children understand the infinite.
I have been re-reading the books. I last read them when my daughter was young, and had forgotten some of the details of the books. It struck me that quite a few difficult ideas are discussed in those books, and explained in such a way that one can begin to understand them, even if one is quite young.
I believe, the turn around in Lewis life was the death of Joy.. his wife (Shadowlands).
His writing became deeper & that of a committed Christian.. not an intellectual Christian.
I had a few classes in seminary that dwelt with Lewis.. first time I had read his work.
He is a deep writer.. the children's books are pretty easy but the adult ones.. well, one needs to experience them.