In terms of interesting characters, The Screwtape Letters by C. S. Lewis.
In terms of life-changing, there are too many to choose from. Maybe my first choice would be The Shack by William P Young but that might not be for everyone.
In terms of books everyone should read, well, there’s the obvious answer. As writers, we should have at least a passing understanding oft he themes and images of The Book of Revelation. Do not underestimate the impact it has had on horror and supernatural thriller stories as well as our culture as a whole.