Can anyone recommend a good translation of the principal Upanishads, preferably a book I can order. Thanks.
An excellent translation is by the combined efforts of the Irish poet Yeats and Sri Swami Purohit. It's called The Ten Principal Upanishads.
In fact Yeats relates that he was moved to make the translation after hearing great things about the Upanishads from his Indian friends but when leafing through the then translations available in English, he thought that if what they said was true, then the beauty of the original hadn't come through. Hence the combined effort by Sri Swami Purohit.
It's also a short book. Out of the 103 canonical Upanishads, the ten principals ones are:
In toto, about a hundred and fifty pages as I recall and was first published in 1938.