"Gospel Writing" by Francis Watson is an excellent book that examines (across 600+ pages) the process by which the Gospels were written and the process by which they came to be accepted as canonical (as Holy Writ).
That's really a main point of the book - that there were many written Gospels used and quoted by early Christians and the 4 we have today were only accepted as 'word for word' inspired thru time as believers came to think of them that way.
A quote: "No religious, philosophical, or literary text enters the world with the label "canonical" already attached. Canonical status is a matter not for authors but for readers; it arises not from composition but from usage." (page 3)
He provides evidence that Matthew was written because many Christians weren't happy with Mark's abrupt, choppy style. Luke put together a Gospel that often interprets Jesus' words instead of just quoting them - and lots of early Christians had real problems with John and Revelation.
Some early comments about Mark were especially stunning - as with the claim that Mark put it together and Peter didn't endorse it or condemn it (!!! whatever, I guess!)
The only reason we have a fixed (and semi-coherent) canon of scripture today is because some church leaders 'kicked a$$ and took names' to make sense of it all and keep the Really Weird stuff out. Like the (very early) Gospel of Thomas..... And they had to 'duke it out' to get some stuff accepted like Revelation. It was a social or political process in the proto-catholic church, not books handed down from heaven.
Remember one thing: if apostasy took off with the death of the apostles, then the "apostates" selected the books that make up the Bible - and there were lots of books and Gospels to choose from.
metatron