Bookverse is the universe presented in a literary version of a fandom. Since, in most cases, the textual form was there before any adaptations, movie, video game, or otherwise, for a 'baseline' of canon many people prefer the books.
Due to discrepancies that must be made for one reason or another, bookverse and movieverse often differ. These differences are sometimes small (Lord of the Rings, in which certain elements were left out or added), and sometimes big (The Bourne Trilogy, which was stripped of just about everything in terms of plot and some characters, and completely rebuilt to be relevant to the current era). This is why the distinction is important.