Which Ancient Indian literature deals with concepts about origin of universe, death and birth, material and spiritual world?

Upanishads- deals with concepts about origin of universe, death and birth, material and spiritual world, which are written in poetry and prose, as expressions of philosophical concepts. Earliest Upanishads are Brihad-Aryanaka and Chanddogya. They explain the highest thoughts described that can be realised by a man, according to ancient sages, in simple and beautiful imagery.