Neither the Old nor the New Testament were dropped from heaven, but were part of a process of development and acceptance as inspired. The Jewish books began with the law, the first five books of the Bible. For Christians, the first books accepted as inspired were the Gospels, although they were not the first written.
Many ancient texts of both Testaments exist. Very early the Old Testament was translated into a variety of languages such as Greek, Syriac, Latin. Today 83 Old Testament texts are known to exist in Old Latin and 171 New Testament texts, some from the second century.
Initially, texts were written on papyrus, made from reeds which grew 12 to 15 feet tall and parchment from the skins of animals. Words were not separated (sometimes, to save space) and there was no punctuation. Although all texts were hand-copied, many Bibles were available. With the invention of printing, translations and copies became much more numerous.
The original books of the Old Testament were in Hebrew. However, many Jews who were living or had lived in other areas where the language was Greek had lost the ability to read their Bible in Hebrew. So it was translated into Greek. This version is called the Septuagint from the legend that 72 scholars working independently translated it exactly the same way.
The deutero-canonical books (accepted secondarily but equally) were written in Greek in the period around 150 B.C. They are: Tobit, Judith, Wisdom, Sirach, Baruch, 1 and 2 Maccabees and parts of Esther and Daniel. They were included in the Septuagint but never found their way into the Hebrew Bible. Why? Possibly because of the animosity between Jews and Greeks, as well as between Christians and Jews.
In the fourth century, the pope commissioned a scholar, Jerome to revise the Latin text of both the Old and New Testaments used by the Church. With the best texts he could find, Jerome began his long and arduous task in Rome and continued in Bethlehem with Paula, a member of a religious group of women. Both of them are recognized in the cave in Bethlehem where they worked.
The Septuagint was the version early Christians used as evident by quotes of the New Testament authors and the early Christian fathers. It continued to be used by the Church. In the 1500s, the Protestant Reformation rejected Catholicism, as well as the Septuagint and Protestants adopted the Hebrew Bible without the deutero-canonical books.
For centuries, Jerome's Vulgate, was the official text. The 1943 encyclical Divino Afflantu Spiritu recommended translations from the original Hebrew and Greek texts rather than translations from earlier translations. Many new translations followed.
Some of these are almost paraphrases of the Bible and their purpose is to make it more easily understood. Others adhere to the translation of the wording even though sometimes, its meaning may be ambiguous. Still others are somewhat in-between.
We have ecumenical translations accepted by Catholics and Protestants. In these Bibles, the disputed books are there, usually in a separate section. The New American Bible is used in the liturgy in the United States. Canadians used the Jerusalem Bible but now use the New Revised Standard Version, an ecumenical Bible, in the liturgy.
Technically, the newer translations are more accurate since scholars have used the oldest and best texts from which to translate. In addition, discoveries of ancient texts of various kinds in recent years has enabled scholars to better understand the way words were used by ancient writers, thus providing a more accurate translation.
Which text is best for Bible study? Some are easier to understand. Some older versions are more poetic and in a more beautiful style. Using more than one translation can be somewhat confusing, but it can provide a better understanding of the text by the different words selected by the translators to convey the meaning of the same text.
Remember no translation is perfect, as each language has its nuances. That's why even in our ordinary discourse, we adopt words from other languages because sometimes no English word quite contains the full meaning of the word in the original language. Although one specific version of the Bible is chosen for liturgical purposes, a variety of texts are permitted for study and private reading/reflection. However, Bibles printed under Catholic auspices and study Bibles usually contain explanatory footnotes which facilitate understanding.