The Taittiriya Upanishad is a Sanskrit text. It is one of the older, "primary" Upanishads commented upon by Shankara. It is associated with the Taittiriya school of the Yajurveda.
Taittiriya Upanishad by Vidyasankar S
Mundakopanishat by MLN Murthy
Ch 4: Mandukya Upanishad by Swamini Atmaprakashananda
Taittiriya Upanishad by Babaji Bob Kindler
Brihadaranyaka Upanishad - Madhukanda by Acharya Sadaji