Star Dust:The Bible. There is not a more important book in all the world.
OC! And after that, I might say different things for different people. Boys and young men should definitely read some G.A. Henty books. (So should young ladies!:):):)
All of A.W. Tozer's books, but in particular, The Pursuit Of God.
All of the books from VF that I've read I would definitely recommend also.
I read The Scottish Chiefs, and I agree.
Besides the Bible, obviously, the Silmarillion and Beowulf are must reads before death.