I believe that the Bible is the word of God and is the greatest text ever written. It is 100% accurate and reliable. That being said, are there any other inspired texts that were written in the first century that one can look at and research and that are reliable? I’m asking about other eye witness accounts outside of the Bible that were written down.
Here is the bottom line: I do not know! Scholars have speculated about a proto-gospel which they call source Q. They argue that it was this source which was used by both Mark and Matthew in putting together their gospels. They even speculate as to what specific material was in this document by an unknown author that they call Q. The existence of other written source material has also been speculated. However, this is simply speculation. We can only go on what we have evidence for and the simple fact is that there is no mention in any ancient document of other gospels written in the first century. Neither do we have evidence of other letters which received serious consideration for inclusion in the Bible (there are some documents from the very late first century such as Didache and the Letter of Clement of Rome, but these are later documents and were never considered to have apostolic authority). Obviousy there were oral sources about Jesus. Were there other written source materials used by Mark, Matthew, Luke and John? Probably. Surely someone wrote down what had happened. Were these sources perfect–inspired? We simply cannot know and speculation is just that–speculation. Such speculation has little practical value.
What about Paul? Did he write letters other than those we have in the New Testament? Surely he wrote other letters!!! Might they also have been inspired? I am sure he wrote other letters and it is entirely possible–I would argue even likely–that some of them were inspired, as Paul was an inspired teacher. Scholars argue that there was a third letter to the Corinthians, for example. However, the simple truth is that we have no other letters of Paul and not even ancient sources which mention such letters. So, we will have to settle for simply saying we do not know.
What about other apostles? Did they write inspired letters to believers or to the church in general? I suppose probably that they did, but without any such documents and without any recorded mention of such documents, we will have to leave the question where it is, which is in the realm of speculation.
By faith, I believe that God saw to the collection of the writings we call the New Testament. I believe that God chose which documents we need. He chose that four gospels would be written and accepted into the New Testament because we needed four independent witnesses to the life of Jesus. Your question is a fun one, but it will probably remain permanently as a fun academic question with no real importance to us as Christians.
Sorry that is all I have to offer,