I recommend listening to '071: How To Build An Inner House of Peace Amid A Chaotic World - Juan Mascaró' first if you haven't already.
Chandogya Upanishad is one of the most cited texts in later Bhasyas (reviews and commentaries) by scholars from the diverse schools of Hinduism, with chapter six, verse 8-16 containing the famous dictum Tat Tvam Asi, Thou Art That. And the Chandogya is one of the oldest Upanishads, often listed as the 9th.
Becoming Antifragile