Compare to Revelations 10:8-11. In those verses the metaphor of the book is used as the instrument of the calling of the prophet to prophesy (see Revelations 10:12). The parallel here makes this event the likely point at which the call as a prophet comes in the revelation.
The book as a symbol in the revelatory experience is not limited to the Book of Mormon and Revelations. Ezekiel is asked to eat a scroll (most parallel to the Revelations account, compare Ezekiel 2:9-10 and Revelation 10:8). Other examples are found in 1 Enoch and the Vision of Hermas ("The Book of Mormon and the Heavenly Book Motif" in Reexploring the Book of Mormon, ed. John W. Welch, Deseret Book Company 1992, pp. 26-28).