The book which Nephi saw was the Old Testament, and more especially the Law, also called the "Torah," as given to the world by Ezra.
Ezra was one of the most prominent Jews in the Babylonian captivity, a descendant of Aaron and in all probability born in Babylon. It was through his influence, humanly speaking, that the return of the exiles was accomplished. (Ez. 7:6 and 28) He led, personally, the second great company to Jerusalem.
Jews and Christians alike regard Ezra as one of the great prophets of the Lord. Among the Jews there is a saying, something like this, "If the Law had not been given by Moses, Ezra was worthy, and by him it would have been given. And Christian authors, such as Irenaeus, Tertullian, Clement of Alexandria, and others actually thought that the Old Testament had become lost during the Babylonian captivity, and that it was restored through Ezra, by divine revelation. Modern scholars do not accept that view, but it is thought that he collected copies of the sacred manuscripts extant, and that he, and other learned men, by scholarly criticism, determined the text and the canon. They accepted twenty-two books as divine. They divided them into three parts, the Law, the Prophets and the Hagiographa or Psalms, a division to which our Lord refers as follows, "All things must be fulfilled, which were written in the law of Moses, and in the prophets, and in the psalms, concerning me." (Luke 24:44) It is also thought probable that Ezra and the other members of the governing council, such as Haggai and Zechariah, edited the manuscripts, and added such passages as could not have been written by the authors of the sacred books themselves, as for instance the account of the death of Moses, etc.
In the book of Nehemiah there is a very impressive account of the acceptance of the Word of God by the people, that may be compared to a similar scene at Sinai, when the Law was given through Moses. (Neh. 9:3)
This was the beginning, as far as known, of the Jewish divine services, with prayer, Bible reading and preaching from Scripture texts, a form that has been perpetuated in Christian churches, and which should never be abandoned, for the obvious reason that it is the word of God that is the life of the Church.
We can now understand why the Prophet Nephi sees the ancient Scriptures proceeding from the "mouth of a Jew." His prophetic vision was fulfilled in the year 444 B.C.