You need to pick all empty doors (ie. the black corridors) until you come to a door with a symbol ON it only, and then you go through that door. It took me like 20 minutes to get to the library though.

If you try to go through any door with a symbol OVER it, you'll get stuck in some kind of trap, or your pet will look like a pile of sand for a bit.

Does anyone remember how many pieces the tablet originally had? I did manage to pick a scroll in the library and supposedly that helps fill in your tablet. But I only have 4 pieces in my tablet; I don't remember if that's the number I started with or not.