I found this to be a very helpful discussion of the Gospel of John. I've owned it and used it for years, but hadn't read through it, which I did this month as I've begun a sermon series on John that will last for a few months.
This is not a commentary, which explicates each chapter of the Gospel. Rather, it is an introduction to the gospel discussing its themes, characters, plots, images, style, authors/editors, audience, etc. Which means that if you are wanting to look up something about a particular passage, you have to jump around a lot, using the index. But I've still found it very useful in sermon preparation.
Carter believes that the question "What is the gospel according to John?" can be answered with:
"The good news according to John is that Jesus is the definitive revealer of God's life-giving purposes and that this mission continues in and through the alternative community, the church, an antisociety that is sustained by the Spirit, or Paraclete, in a hostile world until God's purposes are established in full."
"Jesus as revealer of God's life-giving purposes" preaches very well.