More precisely, there were not two "Book Worlds," but two "sides":
Charlotte Mont-d'Or's "Library" and "Book World" from the original timeline had been integrated into one, forming a strange space with two sides. Entering from the front led to a real space that could store objects and people, while entering from the reverse led to a world of pure consciousness. When Mousse entered it himself, he found that his body had been left outside, so naturally, this went double for anyone else.
The distinction between front and reverse was not primarily because of Mousse's old notion that "matter was primary and consciousness secondary," but because of how these two "worlds" were connected: entering from the front allowed one to find the entrance to the reverse side deep within, but entering from the reverse side revealed no entrance to the front. Even Mousse could only use his highest authority to enter the front directly.
Perhaps it was because Mousse's Book World was unique—or one of only two—and existed independently of real books, with books serving merely as keys to open its door, or rather, as the door itself. Unlike the Book-Book Fruit ability Charlotte Mont-d'Or had developed in the original timeline, which could create one for every book, having fewer naturally made it more "refined."
Or perhaps it was mainly because Mousse's spiritual power and development of his Devil Fruit ability far surpassed Charlotte Mont-d'Or's in the original timeline.
Whether its front or reverse side, the power of Mousse's Book World was astonishing, nowhere near comparable to Charlotte Mont-d'Or's "Library" and "Book World" from the original timeline.
Take the "front" side, for instance. First of all, its space was countless times larger. Compared to this enormous cube, which seemed to have sides nearly ten kilometers long, Charlotte Mont-d'Or's "Library" from the