When we see a book, we tend to think that a single author probably wrote it, but that is a modern assumption. Ancient writers tended to reuse older, traditional material, changing it as necessary to meet their needs.
Several examples of this practice are evident in documents from the ancient Near East. One such example involves a story called the Gilgamesh Epic. Archaeologists have discovered four different forms of this story: Sumerian, Old Babylonian, Middle Babylonian, and Late Babylonian. Later writers took older forms of the story, added material, deleted material, and changed material. The resulting versions differ significantly from one another.
In the Hebrew Bible, the author of Chronicles uses the earlier Samuel and Kings but seriously reworks it to fit his agenda (a uniformly positive portrayal of David and Solomon, exclusive interest in the Kingdom of Judah, emphasis on prophetic speech, strong doctrine of retribution). For example, Chronicles omits any mention of Saul prior to the account of his death in battle, omits the story of David and Bathsheba, omits references to Solomon's foreign wives, ignores kings of Israel unless they come into contact with kings of Judah, explains Manasseh's long rule by claiming that he repented of his earlier sins, and explains Josiah's early death by claiming that he ignored the prophetic word delivered by Pharaoh Neco.
Another Old Testament example of the reuse of older material in a new context involves the books of Daniel and Esther. The Greek form of these books (the LXX) includes a significant amount of additional material. The Greek Daniel includes stories about Susanna, Bel and the Dragon, and the prayer of Azariah from the midst of the fiery furnace. The Hebrew form of Esther never mentions the name of God. The Greek version makes up for this deficit, mentioning God frequently in its six long additions to the Hebrew version.
In the New Testament, the author of the gospel of Luke explicitly mentions the fact that he is making use of earlier gospels which have already been written (Luke 1:1-3). He certainly used the gospel of Mark, which was the earliest of our canonical gospels to be written. Most scholars think he also used a "sayings gospel" called Q (from the German word Quelle, or "source"), because he shares so much material in common with Matthew, but not found in Mark.
The Dead Sea Scrolls contain many examples of reworked biblical material, called "rewritten Bible" by scholars. The book of Jubilees, the Genesis Apocryphon, and the Temple Scroll are all examples of rewritten Bible material. For example, the book of Jubilees reworks the material from Genesis and early parts of Exodus, and it attributes the whole work to Moses. Another example of rewritten Bible is the Diatessaron, a second-century harmony of the gospels written in Syriac, or perhaps Greek.
We tend to think of scribes as people who simply copied the text that lay before them, but scribes were often much more involved in the development of the text, particularly, but not exclusively, when the text being copied had not yet achieved canonical status. In some ways, when a text was considered authoritative it was more liable to certain types of scribal modification, such as explanatory additions, harmonizations, and theological corrections.
Scribes involved in transmitting the Hebrew Bible often inserted traditional material into the text they were transcribing. For example, there are two accounts in 1 Samuel 16 and 17 of David being introduced to Saul in the MT. The LXX omits 1 Sam 17:12-31, 55-18:5. The MT probably contains additional traditional material that was added at some point after the original composition began to circulate (it was not present in the Hebrew version that the LXX translators used).
Scribes sometimes added material intended to clarify something for the contemporary audience. For example, the explanation "prophets were called seers in those days" (1 Sam 9:9) is such a clarification.
As the text began to be considered authoritative, passages that the scribes considered theologically difficult were sometimes modified. For example, in Jdg 18:30, the name "Moses" is changed to "Manasseh" in order to protect the memory of one of Israel's most important personalities (this correction is marked in many Hebrew manuscripts by a raised letter "n" in the text, an indication that something unusual has happened to the text at that point). Another example is Job 2:9, where Job's wife urges him to "bless God and die" in the MT, but the LXX retains the original "curse God and die." A third example is found in 1 Sam 3:13, where Eli's sons are criticized for "blaspheming themselves," which makes no sense; the original read "blaspheming God," a difference of one letter in Hebrew. Jewish scribal tradition preserves a record of many such scribal corrections, but by no means have records of all changes been kept.
In the New Testament, extensive material (i.e., one sentence or more) is added most frequently in the gospels. The ending of the Lord's Prayer in Matt 6:13), "for thine is the kingdom and the power and the glory forever, amen," is such an addition to the original. Luke's version of the Lord's Prayer (Luke 11:2-4) is shorter than Matthew's, but many manuscripts add material to make it conform more closely to Matthew's version. A third example is the story of the Woman Caught in Adultery (John 7:53-8:11), a bit of oral tradition that was added to the Gospel of John (and in some manuscripts, to Luke!).
Theologically-based modifications also occur occasionally in the New Testament. In John 1:18, the phrase "only begotten God," which appears in several manuscripts and modern translations, is probably a theological modification to the original "only begotten Son" that was intended to emphasize the divinity of Christ. In Mark 1:41, most manuscripts say that Jesus looked "with compassion" on a leper before cleansing him, but it is possible that the original reading says that Jesus looked "with anger," a reading preserved in several witnesses. If the latter reading is original, it was changed to preserve a gentler image of Jesus.
One of the most obvious and commonly attested types of modification of the scriptural text was the addition of material at the end of a book. Examples of appended material abound in the Bible. For example, the first 16 chapters of the book of Judges deal with various charismatic rulers (traditionally called judges) of the tribes of Israel, but the book ends with two sections of material (Jdg 17-18 and Jdg 19-21) that, though set in the same general time period, have nothing to do with charismatic rulers, but deal with incidents involving isolated tribes. Another example involves the book of Samuel (1-2 Samuel are one book in Hebrew) and accounts of David's adventures. The logical end of the book comes in chapter 20, after David's kingdom has been restored to him following Absalom's rebellion. 1 Kings 1, the story of David's old age and his appointment of Solomon, follows logically at this point, but someone has added chapters 21-24 at the end of Samuel, a collection of words and deeds of David that were apparently too good to omit from the book.
Additions at the end of prophetic books are common as well. A short, one-verse addition appears in Hos 14:9, where a wisdom saying has been appended to the book. A much longer addition appears in the book of Isaiah, where the material related to the eighth-century prophet Isaiah the son of Amoz ends in chapter 39 and material from more than a century later appears in chapters 40-66 (see, for example, contemporary references to Cyrus the Great, king of Persia, who lived in the sixth century B.C.E., in Isa 44:28; 45:1). Other Old Testament examples of additional material added to the end of a book include Prov 30-31; Eccl 12:9-14; Jer 52; Zech 9-14; and the entire book of Malachi, which appears to be an appendix to the Book of the Twelve (i.e., the twelve Minor Prophets).
In the New Testament, scribes confronted with an original ending of Mark in 16:8 added a longer ending (Mark 16:9-20), supplementing the surprisingly short original ending with traditional material, much of it taken from the other gospels. The Gospel of John originally ended at 20:31, but after the author's death his followers added chapter 21.