We practice this unknowingly

One book.  Bhagwad Gita.  Many great saints have commented on it.  Yogananda, Gyaneshwar, Shankaracharya.  

Perhaps these are the most authentic commentary with each one taking a slightly different view.  All correct.

So in this verse, Shankaracharya Maharaj tells us to give ourselves up to God.  With an unwavering mind.  

Then he gives us the secret – the mind will wander out because of the senses.  You try to go to God but you hear something.  Or you see something.  And you break your connection.  This happens every split second.  If only you could extend that unwavering attention for 6 seconds, you will have a great experience

Now, we try to do this unwittingly.

We go to the temple.  We close our eyes? Why? To block the mind from going outward and stop it from wavering.  We say prayers the same way.  And knowingly or unknowingly, everyone has an intuitive knowledge of this and the knowledge that God is within.  Not outside