There might be some initial struggle and uncertainty, depending on the student, but once the truth of nonduality had been glimpsed, it became obvious that it was always available—and there was never any doubt about how to see it again.
Waking Up: A Guide to Spirituality Without Religion
It is a 2014 book by Sam Harris. Harris discusses a wide range of topics including secular spirituality, the illusion of the self, psychedelics, and meditation. He attempts to show that a certain form of spirituality is integral to understanding the nature of the mind.