Here’s an excerpt from the Roebuck’s Introduction to the book:
The Upanishads are surely among the world’s most influential creative works. Not only did they play a large part in shaping Hinduism as it is today, but the debates that they helped to initiate also influenced, either directly or by reaction, the development of the other South Asian religious traditions, including Buddhism. In the last two centuries they have also begun to influence religious and philosophical thought outside Asian cultural areas. Probably at least half the people in the world have been affected in some way by the ideas of the Upanisads.I have always spelled the word as “Upanishads,” but Roebuck is using “Upanisads”.