This Upanishad belongs to the Atharva Veda. As the name of the Upanishad suggests, the teachings revealed in it are such as to leave its students clean, like a shaven head (mundaka), rid of the veil of ignorance. The lucid translation of the text along with Shankaracharya's commentary on it is an additional aid to comprehend the profundity of the subject.
Discriminate within yourself between the real and the unreal. Have you not read the Vedanta? Even when you sleep, keep the sword of discrimination at the head of your bed, so that covetousness cannot approach you even in dream. Practicing such strength, renunciation will gradually come, and then you will see the portals of heaven are wide open to you.
-Swami Vivekananda