Nisargadatta MaharajThe sense of being, of 'I am' is the first to emerge. Ask yourself whence it comes, or just watch it quietly. When the mind stays in the 'I am' without moving, you enter a state that cannot be verbalized but can be experienced. All you need to do is try and try again.

Nisargadatta Maharaj

Advāita Makaranda
Nectar of Non-Duality, by Lakshmidhara Kavi

Reference Materials:

Video/Audio Lectures

Swami Paramarthananda
7 classes (~90 mins each)

Swami Dayananda Saraswati
33 classes (variable lengths)

Advaita Makaranda - Class #1 - 1984 Spiritual Camp - Swami Dayananda

Swamini Satyavratananda
12 classes (~55 mins each)

Swami Viditatmananda
10 classes (search for "Advaita Makaranda" on the linked page)

Swami Tattvavidananda
7 classes (search for "Advaita_Makaranda" on the linked page)