The so-called “missing,” “lost,” or “forbidden” books of the Bible are the many writings which no Jewish or Christian group has ever considered inspired text. Only books considered divinely inspired have the guarantee of inerrancy (freedom from error).
Most of these “lost” texts were written two to three centuries after the apostles were long dead and buried, and their authorship and authenticity are greatly disputed.
The Protestant tradition calls these books in the Old Testament Pseudepigrapha, from the Greek meaning “false” writings, in contrast to what they label Apocrypha, meaning “hidden” writings. The Catholic Bible calls these lost books deuterocanonical.
Some examples of Pseudepigrapha (Protestant)/Apocrypha (Catholic) would be the books of Adam and Eve, Apocalypse of Moses, book of Enoch, Apocalypse of Adam, Apocalypse of Abraham, Martyrdom of Isaiah, Testaments of the Twelve Patriarchs, and the book of Jubilees.
What Protestants call Old Testament Apocrypha, Catholic and Eastern Orthodox call deuterocanonical: Baruch, Maccabees 1 and 2, Tobit, Judith, Ecclesiasticus (Sirach), and Wisdom. These seven books, plus some chapters in Daniel and Esther, are always in the Old Testament of Catholic Bibles and usually found in Protestant Bibles listed as Apocrypha.
In the New Testament, both Catholic and Protestant traditions merely use the term Apocrypha for the books left out which were never considered divinely inspired, like the Gospel of Thomas, Gospel of Philip, Gospel of James, Gospel of Nicodemus, Gospel of Peter, Death of Pilate, Acts of Andrew, Acts of Barnabas, Passing of Mary, History of Joseph the Carpenter, Apocalypse of Peter, the Revelation of Paul, and the infamous Gnostic Gospel of Mary Magdalene.
These texts were never included in any Bible. Rather than classifying them as lost, missing, or even forbidden, most Christians simply call them Apocryphal.