The book is a piece of argument in narrative form. Its subject is the perpetual virginity of Mary, and it is the earliest surviving text to assert it in the full sense: virgin before the conception, virgin in the act of giving birth, and virgin afterwards. Salome's examination exists to establish the second of those, which is why an otherwise decorous text includes it. Mary's Temple upbringing establishes her ritual purity from infancy; the ordeal of the bitter water forecloses the charge of adultery; and Joseph's age and prior family dispose of the third point, since the brothers and sisters of Jesus named in Mark and Matthew are here his children by an earlier marriage and not Mary's at all.
That last solution had a long career. It is usually labelled the Epiphanian view, after Epiphanius of Salamis, and it remains the standard position of the Eastern Orthodox churches. The Latin West took a different route: Jerome, arguing against Helvidius in the 380s, rejected the older-widower story as apocryphal fable and held instead that the brothers were cousins, which became the usual Roman Catholic position. Most Protestant churches take the plain reading, that they were later children of Mary and Joseph. All three positions are live, and the differences between them begin here.
The text was written in Greek, probably between about 150 and 200; Origen in the early third century knows a Book of James and cites it for Joseph's sons. It survives in more than a hundred Greek manuscripts, far more than any other early Christian apocryphon, and in Syriac, Georgian, Armenian, Ethiopic and Old Church Slavonic versions. The oldest witness is Papyrus Bodmer V, of the fourth century or possibly the third, identified in 1952, whose text is shorter than the medieval manuscripts at several points; whether it abridges an already long text or preserves something closer to the original is disputed.