The Holy Bible, which is always open in a Lodge as a symbol that its light should be diffused among the brethren. The passages on which it is opened differ in the different degrees.Masonically, the Book of the Law is that sacred book which is believed by the Mason of any particular religion to contain the revealed will of God; although, technically, among the Jews the Torah, or the Book of the Law, means only the Pentateuch or five books of Moses. Thus, to the Christian Mason the Book of the Law is the Old and New Testaments; the Jew, the Old Teastament; the Mussulman, the Koran; to the Brahman, the Vedas; and the Parsee, the Zendavesta.
The Book of the Law is an important symbol in the Royal Arch degree, concerning which there was a tradition among the Jews that the Book of the Law was lost during captivity, and that it was among the treasures discovered during the building of the second Temple. The same opinion was entertained by the early Christian fathers, such, for instance, as Irenaeus, Tertullian, and Clemens Alexandrinus; "for," says Prideaux, "they(the Christian fathers) hold that all the Scriptures were lost and destroyed in the Babylonish captivity, and that Ezra restored them all again by Divine revelation." The truth of the traditon is very generally denied by biblical scholars, who attribute its origin to the fact that Ezra collected together the copies of the law, expurgated them of the errors which had crept into them during the captivity, and arranged a new and correct edition. But the true or falsity of the legend does effect the Masonic symbolism. The Book of the Law is the will of God, which, lost to us in our darkness, must be recovered as precedent to our learning what is TRUTH. As captives to error, truth is lost to us; when freedom is restored, the first reward will be its discovery
Masonic Encyclopedia