The classic study of The Four Gospels by B. H. Streeter is available online. I am linking to the chapter on the end of Mark’s Gospel because of my longstanding interest in that subject. Streeter drew the conclusion that John 21 was the lost ending of Mark. I think that putting it that way is problematic, but I do think that John 21 reflects knowledge of a lost ending of the Gospel of Mark, knowledge of which is also reflected in the Gospel of Peter.
I’ve blogged about this before, as well as discussing it in an article for The Bible and Interpretation and in my book The Burial of Jesus: What Does History Have to Do with Faith?.