One of the earliest spiritual books that I have read is entitled "The Third Eye" written by a Tibetan lama called Dr Tuesday Lobsang Rampa.
This is a fascinating autobiography which is certainly an "eye opener" for me at a young age. This book shares a similar theme with another book, the "Lost Horizon" written by James Hilton, which talks about Shangrila, a heavenly get away, high in the Himalayas mountain. While the "Lost Horizon" is a fiction, "The Third Eye" is a factual account.
This book actually helps me to realize there is more to life than materialism, and that human soul exists independently of our body. The books by T. Lobsang Rampa also introduces the law of Karma, or as he puts it "Do unto others as you would have them do unto you".
So, here's a word of thanks for someone who launched my curiosity and started my career as a spiritual seeker .. ;-)