For me it would have to be Revolt Against the Modern World by Julius Evola. No matter where you are in life, no matter how sure you are of your beliefs, this book will at least cause you some mental conflict. I'm sure by now many of you will have either heard of it, or read it, but it's essentially the anti modernist manifesto. Evola makes great arguments for hierarchy, against secularism, against female liberation, and essentially a return to older more Pagan values. The book is very critical of Christianity, but Evola argues his points persuasively, with many of his arguments being hard to deny. The book certainly hasn't made me a Pagan, but it's certainly made me think, so I'd highly recommend it.
What book would you choose?
The bible. The only reason I say this has not so much to do with God and spirituality, but with the truths present in the book itself and how obvious they are once you get into it. It's not a book that tells you rules to live by and forces you into unhappiness, it's a book that ultimately shows you all the mistakes people for thousands of years have made, how to avoid them, and how to live a "safe" life. Think of things like learn, work hard, save money, and surround yourself with good people. That's not there to oppress you, it's good advice.
There's also a lot of the issues there that we're going through today because history tends to repeat. If people were more in tune with that, a lot of messed up things wouldn't be today. Most people are just ignorant, and that's all people need to rule and oppress the masses.