40 to 60 years is not contemporaneous. Writing about Jesus' life half a century after his death during that time would be like writing about JFK today without the aid of any primary sources, and only rumors, fabrication, and work arounds. Witness testimony is suspect even today with the aid of modern technology. 40 to 60 years is a long time, especially given the time period.
The writers of the gospels were tasked with fitting the square peg of Jesus into the round hole of Jewish theology. They had to bend, mold, and distort, in order to fit their messiah into the long established Jewish theology. Muslim theologians have done the same as Christians with respect to Jesus. Jews reject Jesus because he does not satisfy the requirements to be the Jewish messiah and makes claims that are uncharacteristic of the Jewish messiah (claiming to be god, divine, anything other than just a regular human being).
There are non-Christian sources, which are actually contemporaneous with Jesus, which firmly establish that a Jesus-like figure actually existed.
I think of Paul in The Last Temptation of Christ when thinking of the debate about the historical Jesus:
http://youtu.be/kaUuSJx-VDA
Paul invented the idea of Jesus being a god, he did not invent the human being that actually existed. Jesus was more likely a Jewish rabbi who preached an apocalyptic outlook on the world based on the beliefs of John the Baptist. Regardless, Jesus changed the world (arguably for the better). People kill each other less, they are generally nicer to each other, they care less about believing and more about good works. It is irrelevant which god you worship, it only matters that you are good person.
The Quran is a very good example of literature. It is also biased towards Muslims. It promises heavenly benefits to Muslims, while flatly denying these benefits to all those who reject Islam.