The Principal Upanishads

A detailed study of eighteen upanishads.

There is an extensive and exhaustive Introduction, covering just about everything anyone would ever want to know about the Upanishads.

The texts themselves are presented verse-by-verse, with the Sanskrit text (in Roman letters), a translation, and detailed clarification. There are lots of cross references to other Upanishads, and also to all sorts of other sources. The index is only two pages long, and not really much use.

If you want to do any serious study of the Upanishads, you have to begin here.