As Myr says, this series is a complete classic, and I remember vividly how I was captivated when I first read the stories over forty years ago.
C S Lewis wrote a huge number of other books as well, mostly also with some aspect or theme of Christian theology. My dad gave me one called 'The Screwtape Letters' where a young demon is being coached by the devil in how to tempt humans into sin.
He wrote a science fiction trilogy, quite famous at the time, once again with big themes of religious theology running through them - Out of the Silent Planet, Perelandra and That Hideous Strength. I loved them but they are probably not the style for today's readers.
C S Lewis and J R R Tolkein were members, along with other Oxford University Dons, of a group called 'The Inklings' who gathered, for the most part, to read and review excerpts of as yet unpublished literary works.