The Nine Principal Upanishads are the culmination of vedic thought and contain the essence of the original vedantic teachings. They impart sublime truths about the nature and destiny of mankind revealed by great sages and seers during informal discussions with disciples and spirtual seekers. These upanishads describe the direct experience of transcendence which results in the unity of the individual with the highest consciousness.They also provide methods of mediation to realize the nature of Atman, the supreme soul, and attain the immortal Brahman. The Text includes the original Sanskrit verse,transliteration and translation of the following nine major Prashnopanishad, Kenopanishad, Mandukyopanishad, Mundakopanishad, Ishavasya Upanishad, Kathopanishad, Aitareya Upanishad, Shevatashvataropanishad and Taittiriya Upanishad.
Swami Satyananda Saraswati was a sannyasin, yoga teacher and guru in both his native India and the West. He founded the International Yoga Fellowship in 1956 and the Bihar School of Yoga in 1963. He has authored over 80 books, including the well-known Asana Pranayama Mudra Bandha, recognised internationally as one of the most systematic yoga manuals today. Since its first publication by the Bihar School of yoga in 1969 it has been reprinted seventeen times and translated into many languages.
It's fascinating to see the degrees of philosophical sophistication and I'd highly recommend reading the Upanishads to everyone who likes philosophy. Some background information about Indian Philosophy is needed.