The Kena Upanishad (केन उपनिषद्) is a principal Upanishad and part of Sam Veda. One of the important philosophies mentioned in the book is that the ultimate truth is not a matter of mind, senses and philosophical discussions.
There are two remarkable facts about this Upanishad:
Shankaracharya has written two commentaries on it. It is the only book on which Acharya has written two commentaries.
This is probably the first Upanishad in which the Incarnation of Brahman has been mentioned.
There are at least five Hindi translations and commentaries on Kena Upanishad:
Kenopanishad with Shankarbhashya – Gitapress
Kena Upanishad with commentary of Sripad Damodar Satavlekar. It also has the story from Devi Bhagwat which is similar to one in Kena Upanishad.
Kenopanishad with commentary of Ahitagni Yamunaprasad Tripathi. This also has English translation.
Manaswini vyakhya on Kenopanishad by Shri Balkrishna Shastri
Ken Upanishad With Veer Shaiv Bhashya By Pt. Jagannath Shastri Tailang