Is there any good and authentic source where English version of the Upanishads are available.
Well, I can only tell about the method that helped me. There could be other ways as well. It is a subjective choice. It might help you or might not.
What I realised in the process, that directly reading Upanishads at the first go was not very helpful. Upanishads are relatively complicated to understand for a newbie. But the best part is the concept of the Upanishads are explained in an organised way be later Vedantins, which is more accessible to newbies. So my roadmap was this -
First read the central concept of Upanishads. I read this book. Advaita Vedanta: a philosophical reconstruction by Eliot Deutsch. This book is exceptionally organised and gives a concise introduction to the subject. He was the professor emeritus in the University of Hawaii on Indian Philosophy. Link - Advaita Vedanta: a philosophical reconstruction
Next, I read Upanishads with commentary. Without commentary, it would be impossible to get anything meaningful out of it. One of the best translation and commentary on all principal Upanishads is "The Principal Upanishads by Prof. Sarvepalli Radhakrishnan. He was the Professor of Eastern Religion and Ethics at the University of Oxford. When it comes to the order of the Upanishads, read the shortest one first! Start with Isha Upanishad. Link - The Principal Upanishads
Next, I would suggest you read the "The Philosophy of the Upanishads" written by Paul Deussen, Professor of Philosophy. This book is lengthy but explains the concepts in a detailed way. It is one of the classics. You can get the book from this link - The Philosophy of the Upanishads
The Ten Principal Upanishads was compiled and translated to English by Shree Purohit Swami and W.B.Yeats .
download option— http://www.holybooks.com/the-ten-principal-upanishads/
Mukhya Upanishads , also known as Principal Upanishads
- Īśā , Yajurveda
- Kena , Samaveda
- Kaṭha , Yajurveda
- Praṣna , Atharvaveda
- Muṇḍaka , Atharvaveda
- Taittirīya , Yajurveda
- Aitareya , , Rigveda
- Chāndogya, Samaveda
- Bṛhadāraṇyaka ,Yajurveda
these ten are the most important and these three conclude 13 major upnishads.
- Shvetashvatara Upanishad
- Kaushitaki Upanishad
- Maitri Upanishad
Translation by Swami Nikhilananda — http://www.vivekananda.net/PDFBooks/upanishads_nikhilananda.pdf
Although there are online reading — http://www.hinduwebsite.com/sacredscripts/hinduism/upanishads/upanishadsindex.asp
Max muller translation online:—http://www.sacred-texts.com/hin/