The scriptures as a whole. In addition to this:
'AS A MAN THINKETH' - by James Allen
This has to be one of the best books ever written. it explores how the mind works and that thoughts preceed actions. It can help you control your life by contolling your thoughts. Specncer W. Kimball, former President of the Church of Jesus Christ of Latter-Day Saints once said this was the BEST book he had ever read next to the scriptures.
Since the advent of computers and email I have actually found myself falling back on my reading (I used to read a lot!) and I hope to not continue that trend.