You flip to the second page of the tome.
This will make sense later, but it is important to always carry an axe (or similar) while traveling through the infinite library.
Although in my travels, I have only seen maybe a dozen rifts, that is still enough to warrant an entry in this book. The first sign of a rift is often the screams of an individual from just around a corner. If you are smart, you would flee at that moment, but the selfless will follow the screams, looking for someone in peril to save. If you fall into the latter category, you will likely walk straight into the rift.
At first, it will be fairly subtle. Perhaps you will run down a corridor only to meet a dead end, then you turn around to see that the structure of the halls has changed. This point marks the last chance you have to escape. You must run in the direction from which you came. If the environment has shifted, you will need to use your axe to break through, smashing any bookshelves that may have appeared and block the exit. If you can manage to do this before the rift has left you completely disoriented, you may be able to break free. Otherwise, you should prepare to meet a grim fate.
As you remain inside the rift, the library will begin to change more frequently. Shelves will grow extra levels. Books will rearrange themselves, growing and shrinking in size. The walls will shift, closing off some hallways and opening others. The floor will curve inwards, forcing you to jump to the side lest your legs get crushed. Doors will appear that lead to nowhere. Rooms will appear that seem to come from completely different environments. The kitchen of a suburban house. The foyer of a museum. The inside of a clock tower. The tunnels of an abandoned sewer. The halls of a morgue.
Trying to escape at this point will only make your death approach faster. Frantically running down the halls as they bend and contort in violent, jerky motions. What were once individual hallways and rooms become a distorted mess of walls, bookshelves, pillars, doors and other random objects, flowing through each other in unnatural fashion. The walls will strike inwards, hitting you from several directions. They will force your body into freakish positions, instantly breaking your bones and tearing your limbs. As your frail being is quickly crushed, your screams will echo through the broken hallways. Someone will hear you from the distance, someone selfless, the next victim of the rift.
You turn the page back to the table of contents.