I have read an enormous amount of spiritual books. The oldest writings known to man are the Vedas and Upanishads. The former are superstitious rites and rituals, but the Upanishads are great. They talk about the states of sleep like dreaming and dreamless sleep. One of them is my favorite is the Katha Upanishad (takes 20 minutes to read).
It is about the one thing that everyone knows exists but pretend does not exist-- death. Mr N gets 3 wishes from the god of death. After the first 2 he then wants to know the secret of life and death and the god of death begs him to pick something else. Finallly he teaces him.
Here are 2 part from it. "When a person dies, there rises this doubt:
'He still exists' say some; 'he does not.'"
"They say yoga is this complete stillness
In which one enters the unitive state."
For a great life changing movie see the comedy
Groundhog Day. The same day keeps repeating itself.
For a life changing short article online, see
What is Enlightenment.