by C.S. Lewis is one of those must read books, it is an excellent presentation of the basics of Christianity, cutting right to the core of what it claims and making a rigorous defense of those ideas. It is is one of those books by Lewis along with The Great Divorce and The Screwtape Letters that I read through every so often.
Mere Christianity was originally a series of radio talks that C.S Lewis delivered on the BBC during World War II. Here is a video with an accompanying doodle of the start of Mere Christianity for your viewing pleasure.