The important part is that every professing Christian should read this book immediately. Not instead of a Bible, but with a Bible. Check Schaeffer's book against Scripture. Then act like it.
At the surface, it's a book about art, contrasting older works with newer ones. If you never get past that, it's still a very good book, and you'll likely learn to good deal about art and art criticism. It doesn't take a lot of digging to find out it's really a book about worldview.
Worldview is basically the lenses through which we look at the world . An American sees the world very differently than an Australian, though there will be similarities as well. A Han looks at the world differently from a Uyghur, though they both live in China. Men and women have different worldviews. In this book, Schaeffer makes the point that the Christian worldview is different than the non-Christian one, and draws that seemingly simple observation out to its logical conclusion: that Christians should be different from other people.
I can't recommend this book highly enough. I really can't.