This is an excellent companion to the UBS Greek NT. It helps the student of Greek understand why the UBS Committee chose one reading over another in the manuscript tradition and relegated others to the apparatus. Metzger basically takes the problems that the Committee wrestled with and concisely notes them and then reflects on the Committes evaluation and resolution of the problem.
A Textual Commentary on the Greek New Testament (Ancient Greek Edition) https://www.amazon.com/dp/1598561642/ref=cm_sw_r_cp_apa_NlwjBb5XXGSMD
Sample pages discussing Jn. 1:18 and “monogenes theos” versus “monogenes huios.”