Kathopanishad comes from the Yajurveda. It is the story of Nachiketas, son of Vajashravas, discussing the important question of Death and immortality with Yama the Lord of Death. This ancient story, mentioned in various other scriptures, is discussed in the Kathopanishad in detail. Kathopanishad is one of the ten principal Upanishads.
As the name indicates, it is an Upanishad that questions everything, including the concept of the Guru and Disciple. A great rishi proclaims to his students that if they abide with him for some time, at the end of that period he would teach them the Truth if he himself has understood it. Adi Shankara has written an excellent commentary on Prashna which is again one of the principal Upanishads.
The Mundaka Upanishad too is one of the principal Upanishads and derives its name from the shaven head of the renunciant. Therefore the central theme of the Upanishad is the Brahman, the Supreme Truth, and the only Reality, and how to understand it. Paraa vidya or the science of Reality is discussed in contrast to Aparaa vidya, knowledge of the illusory, material world. The path to the Truth is described to be akin to walking on the razor’s edge.