The longest, oldest, and one of the most often quoted Upanishads, Brhadaranyka Upanishad unfolds the knowledge of the self through a wide variety of dialogues. It contains the doctrine Neti Neti (Neither this, Nor that). It also offers methods of worship and Upasanas (meditations) that prepare the mind to receive the knowledge. It’s three sections address the Identity of the Jiva, the Identity of the Atman, and Conversations between sage Yajnavalkya and his wife, Maitreyi. Considered one of the more intellectually challenging of the Upanishads, Brhadaranyaka generally is not studied first. But when it is taken up, the beauty of its elaborate analysis of Vedanta can be fully appreciated. Some consider Shankarabhasya on the Brhadaranyaka to be his greatest work.
An Audio Course on Vedanta Texts- Upanisad : Brhadaranyka Upanisad by Swami Dayananda Saraswati.