Of all the great spiritual literature, my personal favorite is the 2500 year old Tao te Ching or Book of The Way. It is a short treatise, and unlike some other religious tomes like the Bible, can be read in one sitting (set a an hour or 2 aside one afternoon).
I discovered the Tao te Ching about 13 years ago, and although lots of things have come and gone in that time, no matter where I am at in my life, when I dip into the great book every year or so, it speaks with perfect sense.
Here is a snippet from section 30.
The Master does his job
and then stops.
(S)he understands that the universe
is forever out of control,
and that trying to dominate events
goes against the current of the Tao.
Because he believes in himself,
he doesn’t try to convince others.
Because she is content with herself
she doesn’t need others approval.
Because he accepts himself,
the whole world accepts him.
~ The Tao te Ching – Lao Tse (Stephen Mitchell translation)