What is the best spiritual book you have ever read?
For me it is the Hidden Words of Baha'u'llah. The preamble goes like this:
HE IS THE GLORY OF GLORIES
This is that which hath descended from the realm of glory, uttered by the tongue of power and might, and revealed unto the Prophets of old. We have taken the inner essence thereof and clothed it in the garment of brevity, as a token of grace unto the righteous, that they may stand faithful unto the Covenant of God, may fulfill in their lives His trust, and in the realm of spirit obtain the gem of Divine virtue.
(Baha'u'llah, The Arabic Hidden Words)
They are brief sayings, but there is a lot there for me when I ponder them. They are also universal spiritual sayings.