The Letters of John (The Pillar New Testament Commentary (PNTC))
Author: Kruse, Colin G.
Publisher: Eerdmans, 2000
The Epistles of John (The Anchor Bible, Vol 30)
Author: Brown, Raymond E.
Publisher: Anchor Bible, 1982
This book is very technical – not easily used for sermon preparation. But it is the go-to technical volume.