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