Those who preach books of the Bible eventually find their way to 1 John; those who preach a lectionary always find their way to 1 John, and there are some fantastic commentaries on the Epistles of John, beginning with:

Surely the most complete, if not also at times speculative, commentary is that of Raymond E. Brown, The Epistles of John (The Anchor Yale Bible Commentaries)

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A new and useful commentary, with a sensitivity to early Christian identity is Judith Lieu, I, II, & III John: A Commentary (New Testament Library)
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A standard that won’t go away: John R.W. Stott, The Letters of John (Tyndale New Testament Commentaries)
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One of evangelicalism’s finest scholars, I.H. Marshall, writes good commentaries and has a rock-solid commonsens:  The Epistles of John (The New International Commentary on the New Testament)

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A fresh approach from Baylor, one rooted in observations of the Greek text, begins with Martin Culy, I, II, III John: A Handbook on the Greek Text (Baylor Handbook on the Greek New Testament)
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It seems unfair to list this commentary last, but here is another really good commentary: S. Smalley, Word Biblical Commentary Vol. 51, 1,2,3 John
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