Namaste
In the following book on Bridharanyaka Upanishad Bhashya of Shankaracharya, there is a long introduction in English by Sri Swami Maheshanandagiri, Sri Dakshinamurti Mutt, Varanasi.
One may not get a very clear idea by reading just this. One may benefit more by studying the Shaakalya Brahmana of this Upanishad in third adhyaya, ninth brahmana along with the Shankara bhashya and Anandagiri's gloss thereon. There are English translations of this too by Swami Madhavananda of the Advaita Ashrama.
One can also study the Vartika of Sureshwaracharya on the above portion of the Bhashyam. This comes with Anandagiri's gloss too. One may also listen to the class of Sri Mani Dravid Sastrigal, in Tamil, of the line-by-line Shankara Bhashya available here:
There are many classes here and one will have to listen to the recordings one by one to take a complete tour of the Shaakalya Brahmana.
warm regards
subrahmanian.v