From the introduction:
A knowledgeable reader can activate the magical ink of a Library book, creating a portal to its world. This process usually takes hours or days of careful browsing and preparation, depending on the size of the portal. You can rush it, opening a small portal in mere minutes, but will damage the book as you do, singeing the pages and making the ink run. The damage is repairable, given enough time and proper supplies. An hour is enough to open a portal large enough for you to enter, but it will completely destroy the book in the process. You can pull items, creatures, or people through a small portal. It is risky and forbidden to pull living things from their worlds — the transition often kills them—but these are desperate times. If you remove a book from the Library, it will lose its magic and become an ordinary book. This includes taking it through another book’s portal.