This is a very informative from a scholar who is deeply knowledgeable about his subject matter - but it's almost impossible to read. To be fair, he notes at the outset that this book isn't for a newbie just starting to explore this material for the first time. OK, but even taking that into account, it's hard to plow through. I often had to re-read what I'd just read - and often the re-reading didn't help one bit.
I came to this book after reading several of Bart Ehrman's books on the New Testament, where he repeatedly noted how informative this was on the Gospel According to John. It is knowledgeable, but entire pages go by where I don't know what he's talking about. One of the best parts in it, is a two-page timeline/overview of the community that produced this gospel and how it likely broke apart.
Brown argues that it started out as a group in a Jewish synagogue. Enough of the terminology used in the gospel has a Jewish background, not a Samaritan or Gentile one. He also believes that the Beloved Disciple refers not to the apostle John, but a man unnamed in this gospel who was present for Christ's ministry and was actually part of this community's synagogue (hence the title of Brown's book). In fact, Brown thinks that the Beloved Disciple was one of two guys who first saw Jesus when he approached John the Baptist.
More Jews joined the group - ones with an anti-Temple bias who believed Christ had made some converts among the Samaritans.
Eventually, the Johnnite community became increasingly strident in their belief that Jesus was the Messiah. This led to confrontation with the rest of the synagogue - resulting in their expulsion. At this point, they doubled down. They'd gone this far with their belief, so they weren't going to back down. This is a reason why the Gospel of John often refers to "the Jews" as the ones responsible for Christ's problems, not just a group of Jews. Here is where they write their gospel.
Their Jesus is different from the other Gospels. Among other things, Jesus pre-existed here. He always existed, well before his birth, an idea you don't see at all in the other gospels. In John, the apostles call him the Messiah and Son of God within days of meeting Jesus, which doesn't happen elsewhere. Christ's statements on his miracles is totally unlike what he says in the other gospels. The Beloved Disciple keeps one-upping St. Peter in this gospel.
Brown argues that you can find traces of 7 groups being addressed here: three groups of non-believers and four different types of believers. First, is the world, that often hates Jesus and his followers. Second, are the Jews. Not all references to them are hostile, though all references to scribes and chief priests are hostile. Third are followers of John the Baptist who didn't join the Jesus movement. Now for the believers. Fourth are the Christian Jews in synagogues who didn't have the guts enough to break with the Johnnite community did. They are called out as cowards (John 9:22-23 and 9:33-38). Fifth are the Jewish Christians who lack enough faith in him. For example, they don't trust about the eucharist (John 6:66). John redeems Mary, but not Jesus's brothers. Is this an intended swipe at James Christ, leader of the Jerusalem bunch? Are Christ's real brothers then the Beloved Disciple's community? Sixth, are the apostlistic churches - y'know, mainline Christian churches. They are better than the previous group, but don't go far enough on things like Christ's pre-existence. Finally, is the Johnnite group.
Was the Jonnite group an actual sect that broke with mainline Christians? Nah. They condemn the second, fourth, and fifth groups in their gospel- but not the sixth group (the mainline group). It was more an internal debate between Christians. They were willing to work with them - but only if they excepted Christ's pre-existence.
Then came the community after the gospel was written. In short, Brown argues that they fell into two camps, who both used the gospel, but interpreted it differently. One group merged with mainline Christianity while the others drifted into gnosticism and the like. In short, the gospel minimized Christ's earthly teaching and earthly presence while maximizing his eternal divinity. So these groups put their emphasis there. Some fell into docetism - believing that there was no true earthly reality to Jesus's earthly presence. Others went into monopythsitism, that he must have only one true nature (earthly or divine), not two. These guys, perhaps a majority of the Johnnite community, fell into stuff like gnosticism. It's worth noting that our first references to this gospel came not from orthodox Christians, but from proto-gnostics in the late 2nd century.
But others were more moderate. They weren't going to deny Christ's earthly existence. And they were willing to cut a deal with mainline Christians, provided that Christ's eternal existence was accepted. And we see some church leaders begin to accept this idea by the early 2nd century. But the late 2nd century, around 180, they were quoting the Gospel According to John (as its reputation as being written by the apostle made it easier to accept).
So there is a lot to chomp on here. In terms of information, this book is a four or maybe even five. In terms of readability, maybe a two. At best.