Nisargadatta MaharajBy renouncing all lesser desires one reach the Supreme State. As long as you are pleased with the lesser, you cannot have the highest. Whatever pleases you, keeps you back. Until you realize the unsatisfactoriness of everything, its transiency and limitation, and collect your energies in one great longing, even the first step is not made.

Nisargadatta Maharaj

Vedāntasara
Essence of Vedanta, by Sadananda

Reference Materials:

Video/Audio Lectures

Swami Paramarthananda
43 classes (~55 mins each) 1, 2, 3, 4

Swamini Atmaprakashananda
22 classes (~90 mins each)

Vedantasara 2