This Upanishad divides all knowledge into two categories. The knowledge that leads to Self Realization is called Para Vidya (Great or Divine Knowledge) and everything else is called Apara Vidya or Knowledge of Material world (wordly knowledge). Shaunaka approaches sage Angiras and asks "Revered Sir, by knowing what everything will be known ?" Angiras replies that Two knowledges should be known, one is Para Vidya and other is Apara Vidya. Knowledge of worldly things is Apara Vidya and that by which Eternal Truth or Akshara is obtained is Para Vidya.
Vasudeva Mananam (Kannada) by Sri V Subrahmanian
Einführung in Advaita Vedanta (German) by Dhyana
Prabodhasudhakara (Kannada) by V Subrahmanian
Part 3 - Learn Sanskrit by Dr. Narasing Rao
Taittiriya Upanishad by Dr.Karnam Aravinda Rao