I would like to ask you about II Samuel 1:18. This verse speaks of 'The Book of Jasher'. What is this book? I have noticed other parts in the Old Testament that also refer to other books. Are the writers referring to other writings of the Bible? Or other writings from that time? Would these writings maybe be written by someone who was not of God, not a prophet? Since we have been told that the Bible is complete, then this book would not be part of God's word, right? But if this is true, then why is it included?
The history recorded in the Bible did not occur in isolation. The people mentioned in the Bible interacted with other people. For example, though the Bible is firm that there is only one God, still the Bible mentions a number of the gods people worshiped both within Israel and in the nations around them (see "Mythology" for a list).
In a similar vein, we sometimes find secular writers being quoted (Acts 17:28; Titus 1:12). This doesn't mean that these quoted writers were inspired. It simply means they happened to say something that was useful in making a point.
"Then David lamented with this lamentation over Saul and over Jonathan his son, and he told them to teach the children of Judah the Song of the Bow; indeed it is written in the Book of Jasher" (II Samuel 1:17-18). The song that follows these verses, titled the Song of the Bow, could also be found in the Book of Jasher. Just as a children's Bible storybook might copy a section of the Bible for a child to learn, the writer mentions that the Song of the Bow could be found in another book. But just as we don't refer to a children's Bible storybook as being inspired, there is no need to conclude that the Book of Jasher was an inspired book. It was just a book that happened to contain a copy of some inspired writing. That same book also contained a story from Joshua (Joshua 10:13).
In the Notes on the Bible, there is a list of other writings mentioned in the Bible. Because they are mentioned, it doesn't mean they were inspired writings.