C. S. Lewis's Masterpiece

Mere Christianity was about Christianity in general, carefully refraining from teaching the doctrine of any particular denomination.

About Mere Catholicism

It explains specifically what Catholics believe and practice. Meant for both Catholics who want a better understanding of their faith and non-Catholics who are curious about what the Catholic Church teaches, the book presents the basic doctrines that Catholics believe and the basic devotions that they practice. It is not a catechism but is written in a popular style.