Most say the author was the apostle John. In William Barclay's Gospel Of John Daily Study Bible he says,
There were two Johns in Ephesus, John the apostle and John the elder. Barclay says it was John the elder (who was actually younger than John the apostle) who actually penned the book based on the mind and memory of John the apostle.
The Introduction to John in the New International Version (NIV) version of the Bible, 1985 says,
"Some interpreters have felt that John's aim was to set forth a version of the Christian message that would appeal to Greek thinkers. Others have seen a desire to supplement (or correct) the Synoptic Gospels, to combat some form of heresy, to oppose the continuing followers of John the Baptist or to achieve a similar goal. But writer states his main purpose clearly: 'These are written that you may believe that Jesus is the Christ, the Son of God, and that by believing you may have life in his name' [20:31]. He may have had Greek readers mainly in mind, some of whom were being exposed to heretical influence, but his primary intention was evangelistic."
Elaine Pagels in her book Beyond Belief: The Secret Gospel of Thomas,
proposes that John wrote his gospel as a refutation of Thomas'
gospel which was rejected, when the New Testament
canon was fixed in 357. Both gospels center their themes on a
higher knowledge available in Jesus' words and message, but Thomas
claimed that light of God is in everyone not just Jesus.
In Barclay's Guide to the New Testament he says,
"For many Christian people the Gospel according to St. John is the most precious book in the New Testament. It is the book on which above all they feed their minds... nourish their hearts, and ... rest their souls.... Many people find themselves closer to God and to Jesus Christ in John than in any other book in the world."
At the Carlisle Baptist study of John they say,
Even the most cursory reading will quickly reveal that John's is a very different kind of Gospel. There are many stories from the life of Jesus that the first 3 Gospels include which John leaves out- nothing of His birth, baptism, or temptations; nothing of the Last Supper, Gethsemane, or His Ascension. But there are also many stories which John includes that are not in the other Gospels- the story of the marriage at Cana, of Nicodemus, of the Samaritan woman at the well, and of the raising of Lazarus. The first 3 Gospels focus much more on Jesus' ministry in Galilee; John focuses almost exclusively on His ministry in Judea. Matthew, Mark, and Luke contain lots of Jesus' teaching but it is usually in short vivid analogies and brief, pithy parables. In John's Gospel, the speeches of Jesus are much more extended discourses, sometimes a whole chapter long. The first three Gospels deal primarily with the events of Jesus' life- they tell a chronological story, but John goes deeper and deals with the meaning behind those events- he tells a theological story. He tells us not only what miracles Jesus did, but what these signs mean, and what they reveal about Christ. For example, all 4 of the Gospels tell us about Jesus feeding the 5,000 with just a few fish and a couple of loaves of bread, but only John gives us the great sermon on Jesus as the Bread of Life (John 6).
Barclay's Guide to the New Testament - William Barclay - Google Books