The question as to which book of the Bible is the oldest is impossible to answer because the ancient world did not have books and authors as we think of them today. In ancient times, there were primarily authoritative speakers who delivered their message to hearers. There were professional scribes who may have written down the message, but the resulting documents were not copied and distributed. A copy may have been stored in an archive somewhere, but the average person would not have had access to it, even if he was able to read. Speaking and hearing dominated the culture, just as reading and writing dominate ours. As a result, ancient people were much less concerned about when a book was written than we are. They would have been much more concerned that the message they heard was from an authoritative source—even if the message had been repeated several times.
The oldest events in the Bible obviously take place in Genesis. We know that God gave the message to Moses, but we do not know when it was written down and became a “book.” Some of it was inscribed on tablets (Exodus 34:1–4), and other parts of it were probably written down as well at the time, but we do not know when they took the “final” written form that we have today. The language of the Pentateuch shows evidence of development and updating (it is not as “ancient” as we would expect if it was “written” at the time of Moses), so the “final” form may not have emerged until many years after Moses, although the message goes back to him and ultimately to God who gave it to him.
Some will point out that the book of Job appears to record events that took place before the time of Moses, and it is thought by many to be the oldest book of the Bible. However, the theological emphases of the book of Job seem to indicate that the concerns the book is addressing are much later, perhaps from the exilic period, and addressed to Israelites. So, while the book may contain information about events that happened long before, the book seems to have been written much later than Genesis was written.
In the final analysis, Genesis certainly contains the oldest information, and the message of Genesis came from Moses who was the first prophet to have his words formalized in written Scripture, so it would probably be accurate to see Genesis, or perhaps the Pentateuch as a unit, to be the oldest book of the Bible.