Dara Shikoh's translations of 50 out of 108 Upanishads is said to be the most authentic source of the Upanishads available to date. Which is the best book available in the market which compiles all of Dara Shikoh's works on the Upanishads?
As far as I can tell, there are no direct English translations of Dara Shikoh's translation. But it looks like in the late 1700's, Anquetil Duperron translated Shikoh's Persian translation into Latin, as described here:
In that year Anquetil Duperron, the famous traveller and discoverer of the Zend-avesta, received one MS. of the Persian translation of the Upanishads, sent to him by M. Gentil, the French resident at the court of Shuja ud daula, and brought to France by M. Bernier. After receiving another MS., Anquetil Duperron collated the two, and translated the Persian translation 2 into French (not published), and into Latin. That Latin translation was published in 1801 and 1802, under the title of 'Oupnek'hat,'
This book seems to be the only English translation available of the Oupnek'hat.