Part of the Krishna Yajurveda, and one of the most popular Upanishads, Katha Upanishad comprises two parts and 120 verses. It is one of the ten Upanishads Shankara commented on, and comes to us in the form of a dialogue between young seeker Nachiketa and Lord Yama (Death). Lord Yama test Nachiketa’s qualifications as a student and finds in him unswering Shradha (Faith), Vairagya (Dispassion), and resolve for the absolute truth. Impressed, Lord Yama teaches Nachiketas that most sacred of al knowledge- the knowledge of the self that is full and complete. This dialogue between Lord Yama and Nachiketas has all the elements we require to understand the self. Lord Yama thoroughly answers Naciketas question about the nature of the self that is doubted even by the gods and is other than dharma and adharma and cause and effect.
An Audio Course on Vedanta Texts- Upanisad: Katha Upanisad by Swami Viditatmananda Saraswati.