It seems that the word "Book" is used to mean a heck of a lot more than bound paper in the realm of things Magiq.
The "Book" of Briars is a spell. Inlaudetus is an era in history called the "Book" of the Hidden.
To me at least, Augernon's tone in this message is a healthy reminder that there is a hell of a lot more to all of this than solving the Fragments.
Thoth keep them, Ma'at guide them.