The beginnings of philosophy in India, by Richard Gotshalk, 1998
I don't think this book has been mentioned on the site before, so I wanted to do so. It's a broad discussion of the state of affairs that preceded the Buddha, and contains translations of relevant Upanisads as well.
chapter 4 | First Phase (1700-1000 BC)
chapter 5 | Second Phase, First Period (1000-700 BC)
chapter 6 | Second Phase, Second Period (700-300 BC)
chapter 7 | The Earliest Upanisads
chapter 8 | Transitional Upanisads
chapter 9 | Second Phase of the Upanisads
"And how is it, bhikkhus, that by protecting oneself one protects others? By the pursuit, development, and cultivation of the four establishments of mindfulness. It is in such a way that by protecting oneself one protects others.- Sedaka Sutta [SN 47.19]
"And how is it, bhikkhus, that by protecting others one protects oneself? By patience, harmlessness, goodwill, and sympathy. It is in such a way that by protecting others one protects oneself.