About the doer and knower During my meditation ,I experience automatically switch on  to breathe.what is this?Is it letting go of doer and knower?
Yes. 

When the mind is quiet the breathing automatically becomes the object of meditation.

There is no need to try to watch the breathing. In fact, trying to watch the breathing is an obstacle to deep progress. 

The Pali scriptures (SN 48.10; MN 118 at the emd) state the meditator makes 'letting go' the object of meditation.