The writer of John is definetly the beloved disciple. And from context clues, we know that it was John, probably the youngest of the 12 apostles
John is a very complex book, despite its simple greek, it plays on alot of words and has lots of interwoven themes going through it. I encourge you to read commentary about it.
When its all said and done, I plan to have read 350-400 pages of verse by verse commentary from the New Interpreters Bible, volume 9