This Upaniṣad belongs to the Atharvaveda. It is a fairly long work of 337 verses spread over five chapters.
A yogi, Nidāgha by name, approaches Ṛbhu, the ‘best among knowers of Brahman,’ for Self-knowledge. Ṛbhu prefaces his teaching by narrating how his father had taught him the ‘best of mantras,’ ‘the mantra of Annapūrṇā’ of 27 letters (aim hrīṁ sauṁ śrīṁ klīm oṁ namo bhagavatyannapūrṇe mamābhilaṣitam annaṁ dehi svāhā), how he had pleased the Divine Mother Annapūrṇā by repeating it day after day for a long time, how he had asked for ātmajñāna or Self-know-ledge from her when she appeared before him, and how he had got it by her grace. The name of the Upaniṣad is derived from this legend.
The rest of the Upaniṣad is just a long and loose discourse on Advaita philosophy in which many of its aspects have been brought in, though not in a systematic manner.
Apart from the topic of ‘bhrama’ (delusion) and how to overcome it, the Upaniṣad also deals with topics like vāsanākṣaya (destruction of tendencies carried over from the previous lives), manonāśa (dissolution of the mind through yogic exercises) and a detailed description of the jīvanmukta (‘the liberated in life’) who never gets affected by the various vicissitudes of life.