Textual Commentaries (note: different from regular "commentaries") are books that provide a commentary on various text-critical issues of the New Testament. These include manuscript variants, word order changes, or differences in conjugation, etc. The library has two main textual commentaries: Bruce Metzger's Textual Commentary, and Roger Omanson's Textual Guide, which is itself an adaption of Metzger's.
A Textual Commentary on the Greek New Testament by 
A Textual Guide to the Greek New Testament by 
A Commentary on the Manuscripts and Text of the New Testament by Handbooks on the Greek New Testament Series
This is a partial series of books on each book of the New Testament, designed as a handbook for interpreters and translators. Each edition will cover major text-critical issues in its book of the Bible.
For example, here is the edition on 1 Peter:
1 Peter: A Handbook on the Greek Text by UBS Help for Translators Series
The United Bible Society published many substantive individual volumes on translation issues in books of the Bible. Some of these editions are older, but still reliable. Use the link above to browse titles in the series.
Example:
A Handbook on the Letter to the Hebrews by