T. R. Śrīnivāsa Ayyaṅgār, and S. Subrahmaṇya Śāstrī. 1938. The Yoga Upaniṣads. Translated into English (On the Basis of the Commentary of Śrī Upaniṣad-Breahma-Yogin), The Adyar Library Series, The Adyar Library Series, 1st edn (Adyar: The Adyar Library)

Worldcat Permalink: http://www.worldcat.org/oclc/663407262

@Book{ayya-yoga,
author = {T. R. Śrīnivāsa Ayyaṅgār and S. Subrahmaṇya Śāstrī},
title = {The Yoga Upaniṣads. Translated into English (On the Basis of the Commentary of Śrī Upaniṣad-Breahma-Yogin)},
date = {1938},
location = {Adyar},
publisher = {The Adyar Library},
edition = {1},
url = {https://www.archive.org/stream/TheYogaUpanishads/TheYogaUpanisadsSanskritEngish1938},
urldate = {2017-06-21},
language = {sanskrit},
series = {The Adyar Library Series},
number = {20},
note = {A second revised edition appeared in 1952},
}

This contains 20 original texts on different aspects of theory and practice of Yogic techniques. Also English translation for these texts are available in separate volume. The translation is standard, but not scholarly.
These texts of have lot of materials in common and also many repettions are there. However, this book is very useful for a student of Yoga. Historically speaking, these may not comprise of ancient Upanishadic texts which are part of Vedas.
Upanishad Brahmendras commentary is not exhaustive, but is useful only to a small extent.
The Kuppuswami Sastri Research Institute, Chennai has published three Yoga upanishad separately after making corrections in the texts published by the Adyar Libray ( Theosophical society, Adyar)Chennai.