Thanks so much for writing to us here at TruthBook with your question about the Bible. Although we are not Bible scholars, we do value the Bible. The Urantia Book honors the Bible, and Scripture; and Jesus used Scripture very liberally in his ministry.
It is my understanding that the Bible as we know it today was compiled from writings that were available from even before the times of Jesus; and sometime around 300 AD, these writings were gathered into one place. Who was responsible for deciding what went in and what got left out, I am not sure. Those "lost books" were ones that were left out, but why, I don't know. Probably some searching in Google might help you...
As for where that Bible information originally came from, I do know that part of it is a history of the Levantine people; it likely was originally oral history that was eventually written down.
As for the rest of it, I can only assume that there have always been those people who had a close connection to God, and who were able to "hear" what he had to say ... people like Isaiah and other prophets. And these writings proved helpful to people of the times, and still retain a lot of relevant truth for us in the 21st century. God was communicating with his children then, and he continues to do so today. The Urantia Book is an evidence of this. Even though many feel that the Bible is the only source of truth, it seems not to be the case. God has never stopped talking to us, if we have ears to hear, and seek his truth with an open mind and heart.
There are many references to "Scripture" in The Urantia Book. You can go to our search feature at the top right of any TruthBook page (search the word scripture) and see for yourself. It is easy to use, and you might be surprised at how often Jesus made reference to Scripture. Even in his day, these writings were considered sacred, but he did not always agree.
Click to read what Jesus said to Nathaniel about Scripture
Your second question about whether we had an existence before we were born here is interesting, and it is a question that has been asked of us before. I am giving you a link HERE where you can read our answer. I hope it is helpful to you.
Thanks again for your question. I hope you'll want to write again; please feel free to do so, anytime!