Well, since you asked about CS Lewis I can't resist responding!

C.S Lewis is an author and person whom I really admire. He was a professor at Oxford (Literature and also.... maths) A real renaissance man. He was a good friend of JRR Tolkiens and he rallied Brits to keep their spirits up during the the war. He's not only famous for the Narnia books, but wrote some very good books, all of which I'd highly recommend.. For example:

Screwtape Letters (a very entertaining book about a young demon (from hell...) who is assigned by the Devil to try to morally corrupt an Englishman. The book consists of his his correspondence with his uncle, a more senior demon who is advising the younger one on the tricks of the trade...)

Mere Christianity: A book about Christianity for people who are not willing to simply swallow the Sunday school stories with no questions asked... Or for an agnostic who is willing to read a pro-Christianity book. It's a traditional Christian apologetic book.

He's written many more, including some science fiction. He has a very pleasant way of using the English language. Very British, very readable.

As for the Narnia books: They are some of the most magical books ever written. Comparing them with Harry Potter should be banned... They are in a completely different league.

The reasons they are so famous are:
1) Great plots, epic saga
2) Great language and style of writing
3) The simple yet very clever Christian allegories that are present in each of the book.

It is possible to read and appreciate the books for reasons 1 and 2 and never even notice 3. That's exactly what I did when I was a kid, maybe 9-10 years old. I didn't even spot the relevance of "The Lion the Witch and the Wardrobe" animation being broadcast on TV every Christmas. Later, I could see the parallels with the Bible - and it had a really strong impact on me.

The books are about Christianity and the Bible, but in a very non-preachy sort of way. Each book illustrates concepts or ideas from the Bible and turns it into an exciting adventure.

Apart from the Disney filmatization, BBC also filmed some of the books in the 1980s. There was a British animated version of the most popular books, as I mentioned. It was shown on TV during the Christmas break all my childhood. BBC has also made audio dramatisations of the books. I liked the Disney version of "The Lion the Witch and the Wardrobe".

The order in which to read the books is:

The Magicians Nephew
The Lion, the Witch and the Wardrobe
The Horse and His Boy
Prince Caspian
The Voyage of the Dawn Treader
The Silver Chair
The Last Battle