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In most cases, the reason those other scriptures are not in the Bible is pretty clear once you read them. They tend to be fanciful extensions of the story, and those who read them concluded they weren't helpful or weren't consistent with the Christian teaching passed down from the Apostles.
But most are available in English. Quite a few were products of the Gnostic branch of Christianity (which primarily got its ideas from a peculiar philosophical group, and grafted on the Christian story). The Christian Gnostics were regarded as heretics from about the third century, but thanks to a fortuitous cache of their early literature in Coptic Egyptian versions, which came to light last century, we now have many of their writings. Look for a collection entitled "The Nag Hammadi Library."
One Gnostic text was even harder to find. A manuscript--somewhat damaged as it was passed around by people hoping it would make them rich--has come to light. Look for "The Gospel of Judas." (And yes, that does refer to Judas Iscariot.)
Quite a few other early Christian texts were well-known in the fourth and fifth centuries, when the New Testament canon was settled. Many of these are available, in somewhat old translations, in a collection called "The Lost Books of the Bible." Among other curiosities, they include "The Shepherd of Hermas" and "The Epistle of Barnabas" (both highly regarded among early Christians, but not accepted as apostolic in origin); "The Gospel of Nicodemus" (also called "The Acts of Pilate"), which later was connected to the Holy Grail story by Robert de Borron; and the "Gospel of the Infancy of Christ" and some material concerning Mary, which was extensively mentioned by Muhammad in the Qur'an.
One item we no longer have is the so-called Q source, which supplied a lot of the material common to the gospels of Matthew and Luke. (But some modern writers have had the hubris to publish purported reconstructions of it.)
Another item we no longer seem to have is the gospel in Hebrew, mentioned by the second-century Christian writer Papias (although we only have HIS material second-hand, through Eusebius) and attributed to either Matthew or Matthias. It's the reason the first canonical gospel is mistakenly named for Matthew, though it clearly isn't the one Papias was discussing. But it seems likely the half or so of Luke that isn't derived from Mark or from Q, and the overall chronology he used, are based on the Hebrew Gospel, in a very direct translation to Greek from Hebrew.
參考: For information on the Hebrew Gospel, see "The Hebrew Gospel and the Development of the Synoptic Tradition" by James R. Edwards.