In chapter 15 of "Jesus and the Eyewitnesses: The Gospels as Eyewitness Testimony", author Richard Baukham expresses his view that John the brother of Zebedee and the Beloved Disciple who appears in the Gospel of John are not one and the same person. In chapters 16 and 17 Baukham states his case in detail, also claiming the the Beloved Disciple, also named John, wrote the Gospel of John, the Epistles of John, and Revelation; he believes that John the Son of Zebedee did not write these documents.
In Chapter 16, "Papius on John", Baukham points out that the Synoptic Gospels list the twelve Apostles and that these lists acknowledge the debt to the twelve for the contents of them. But the Gospel of John contains no such list. To Baukham, this is evidence that the one most responsible for this Gospel's contents was not one of the twelve. Also, the Son of Zebedee is mentioned only once in this Gospel, Jn. 21:2. Baukham also points to the quote of Papius, see here for the quote, where Papius lists the Apostles in the same order listed in John's Gospel, indicating to Baukham that Papius used that Gospel in constructing his document. Baukham believes Papius made a distinction between John the apostle and the Beloved Disciple. I did not pick up that impression reading the quote.
In chapter 17, "Polycrates and Irenaeus on John", Baukham examines quotes from Polycrates and Irenaeus, both 2nd Century church figures. Polycrates was a bishop of Ephesus who was related to past bishops of Ehphesus and knew those who, like Polycarp,had learned from the original Apostles. A tradition that the Beloved Disciple lived in Ephesus in his final years was well established by the time Polycrates was writing. Baukham believes this document shows that Polycrates claimed a blood relation with both the Beloved Disciple and Philip the Evangelist through one of Philip's daughters. I do not make that connection from reading the document Baukham presents in his book. Baukham also believes that this document is evidence that John was a member of the High Priest's family and may have briefly served as High Priest. I find neither Polycrates document nor Jn. 18:16 evidence for this claim. Baukham also states that the tradition linking the Beloved Disciple to Ephesus never labels this disciple and John the Son of Zebedee the same person. According to Baukham, neither does any of the writings of Irenaeus link the two.
There is much information in these chapters that I can not delve into detail because of time constraints. While I am not competent to establish the correctness or lack thereof for much of what Baukham asserts, the language Baukham uses is more in line with speculation rather than assertion of fact. Baukham himself admits that there is no hard evidence to back up either his theories on this subject or those who advocate alternatives. He also states that the majority of scholars attribute all the writings of John in the New Testament to John the Son of Zebedee.
Only one more chapter to go.