Don't attempt to repeat or memorize these pointers. Rather, let them resonate within you. You may even try putting them aside for a while, and then re-reading them. Coming upon them fresh may allow you to see, directly and unequivocally, the awareness to which they are pointing.
Answers questions about nonduality aren't meant to make you feel good or to confirm some philosophical concept of yours. Rather, they are to help you to directly perceive who--or more accurately, what--you truly are.
Ancient texts on nonduality are fine. But you are more likely to feel an affinity with a present-day book or teacher. As laudable as a few of the early classics are, contemporary works tend to be less dense with cultural, historical, and linquistics subtleties. Once your understanding is solid, you will be able to appreciate these early works more fully, separating, as it were, the dross from the cream. Or you may have no inclination to return to them at all. Every personality is different.
There are no grades or levels to our natural state. No person is more "enlightened" than the other. Buddha and Christ's understanding are the same as Nisargadatta and Bob Adamson's. However, each person expresses that understanding differently. And there certainly may be varying experiences before self-realization. But these events do not lead to your cognitizing presence. That can only be done immediately, tout de suite. But most seekers not only long for experiences, they want the exact ones that they have heard and read about from popular spiritual writers and teachers. But such endeavors are unavailing. For experiences, however spectacular, are temporary. Thus, they can never be the essence of who you are.
Nonduality has been given a multitude of definitions through the centuries. You can sieve through the glosses and interpretations at a host of spiritual Web sites. But nonduality proper is simply the direct knowing of your natural state.