
There are two major opinions with regard to the authorship of the Fourth Gospel. The conservative position, held by Carson, Keener, Kostenberger, and others, is that John the son of Zebedee (or someone close to him) wrote/edited the gospel. The more progressive position, held by (the deceased) Raymond Brown and others, is that the author(s) are unknown and that the Fourth Gospel and Epistles of John tell us more about a "Johannine" community than they do about Jesus. There are, of course, mediating positions, but these two extremes are the most common views.
Hengel offers another solution to the question, namely that "John the elder," a mysterious figure mentioned by Papias (as quoted by Eusebius) wrote the Gospel and that over time he was confused with the more prominent John the son of Zebedee. According to Hengel, John the elder was a young man during the ministry of Jesus, taught in Ephesus after the resurrection, wrote the Fourth Gospel, I, II, and III John (and perhaps Revelation), and was one of the last eyewitnesses to pass away (probably at an old age). Bauckham follows this theory and adds that perhaps John the elder was a disciple of John the son of Zebedee, further adding to the historical confusion.
In my opinion, the best part of Hengel's book is his evaluation of the more "liberal" views that the Fourth Gospel is a cut-and-paste collection of anecdotes put together by a series of redactors over a long period of time. Hengel shows that this is extremely unlikely. However, I wish he had interacted more with the more conservative position. The only discussion that I saw in the book was an allusion in a footnote to a tradition that John the son of Zebedee was martyred at a young age (before the Fourth Gospel could have been written) and a reference to a 1962 JBL article written by P. Parker.
I wasn't convinced by Hengel's arguments that "John the elder" wrote the Gospel. It seems to me that if John was prominant enough in the church to have produced 4 or 5 books of the New Testament canon, more steps would have been taken to distinguish him from John the son of Zebedee. The early church went to great lengths to distinguish between James the brother of Jesus and James the son of Zebedee, and between the numerous guys named "Judas." It seems odd that this guy John the elder would fall off of the historical map. (Note also the distinction in the Synoptics between John the Baptist and John the son of Zebedee. If it weren't for the latter John, perhaps John the Baptist would just be known as "John.")
After reading The Community of the Beloved Disciple by Raymond Brown, Life in Abundance: Studies in John's Gospel in Tribute to Raymond Brown edited by John R. Donahue, The Testimony of the Beloved Disciple by Richard Bauckham, and The Johannine Question by Martin Hengel, I am pretty convinced that there is good reason to attribute the Fourth Gospel to John the son of Zebedee.