Nisargadatta Maharaj

Nisargadatta Maharaj is one of the most influential of Advaita teachers. He is famous for the book "I am That".  The essential core of his teaching is that we cannot say with certainty who we are, all we can say for certain is what we are not. Our essential nature is indescribable by the mind for it is beyond the mind.

To this end, it is useful to stop claiming sense objects/perceptions as me or mine. A useful book to start off is this book:

This Pain is Not Mine