John Mark Reynolds has listed the five books by C. S. Lewis that influenced him the most. For what it's worth, here's mine, in order of most influence to least.
(1) The Abolition of Man
Education is always in some way (even a fundamental way) an education of values. The educator cannot escape this fact. Better to own up to it and make the conscious decision for objectivity than to ignore it and be a functional subjectivist.
(2) The Screwtape Letters
The book of Lewis that almost everyone reads first is the one I read last. Still, I was left at the end with a sense of awe at Lewis's ability to describe the human condition in all its twists and turns. And, of course, many parts of it felt like they were written with me in mind.
(3) "Christianity and Culture"
An essay not a book, but nonetheless it has a pithy warning that needs to be kept in mind: "culture is not everyone's read into Jerusalem, and for some it is a road out."
(4) That Hideous Strength
There is so much to think and feel about in this book that it usually overwhelms me when I read it. And when it doesn't overwhelm me, it almost does.
(5) An Experiment in Criticism
For someone who likes to make fine judgments, this book reminded me that for all practical purposes the distinction to make in literature is that between great and everything else.
(6) The Lion, the Witch, and the Wardrobe
One word: jollification.