For today's look at Biblical Reliability, I would look at a piece of evidence that lends credibility to the Gospels' historicity, although one I don't see mentioned as often as some of the others. This argument doesn't touch other Biblical books much, although to an extent it could be applied to most. The argument deals with formulating solutions to problems.
In NT times, as today, there was a great diversity of opinion on how to solve certain diagreements, or address certain questions. When a group at that time wanted to start a new faith, it would have been incredibly tempting to answer those questions dogmatically, via their "official" writings and teachings.
As we see in later NT books, the fledgling NT church had many questions to answer. Should new Christians be circumcised, or not? Can Christians support war? What is the role of women in this new faith community? What is the deal with baptism?
None of these questions are answered in the Gospels, at least not directly. Each of the Gospel writers included text that could be interpreted by those holding a different side to each question listed above. Jesus was described as coming to fulfill the law, but treated the law in a way not found by those thought to be the keepers of the law. So, how do we decide on the circumcision issue? Jesus said to render unto Ceasar, which could mean supporting the Roman military, but He also said, "Blessed are the peacemakers."
We even struggle with some of these issues today. For the leaders of the early church, some of whom (John, Peter, Matthew) had a hand in the Gospels (Mark was Peter's disciple, and it is not a stretch to think Peter provided much of the material for Mark's Gospel), it would have been simple to add a line like, "and Jesus said, 'lo, to follow me one must become circumcised.'" It would have saved them some headaches, and the integration of Gentiles into what was an originally all-Jewish faith was a major headache. But these types of easy answers to pressing questions are not found in the Gospels.
So what does this prove? Like all the arguments I'm putting forth in this series, not much by itself. All this does is indicate that the writers were not motivated by a desire to make their lives easier. The Gospels don't appear to be written as guidelines for a new religion so much as passing on information about the life of history's most compelling subject. Take it for what you will, but I find the lack of easy answers a marker in the Gospels' favor.
A final note. Some of these questions were addressed, if not answered, in Paul's letters. That is well and good, as it would be inevitable that issues of great social import would be addressed by the Apostles at some point. However, this evidence is towards the credibility of the Gospels themselves, and not the NT as a whole. Paul certainly left some questions unaddressed as well.