It's full of interesting things.
Strike a pane of ice or glass sharply, once, and it fractures not into one line but into a webwork. Strike time, break the continuity-of-things with an event, and it, too, fractures. It's just that most beings, or at least most sane beings, are only aware of the crack-path that their own timeself goes down.
This book/sphere/port knows every other path of your life.
What if you had been born male instead of female, or a rabbit instead of a newt; what if you were a Bubble Doll instead of a Vickie; what if that lover had never left you? Read and find out.
But there's something else about the book. It doesn't drive you mad. There are plenty of books that can drive you mad. It changes you. Read for a while, and you may find that your hair has turned blue. Read a while more, and you may find that you have, in fact, turned into a serpent. Read too long and you may be changed beyond recognition -- or you may cease to be, cease-to-ever-have-been. They say.
You can't observe time without changing it, or being changed by it.
The Polychronicon started out in the Specialized Collection of Pararchaeological Artifacts, but they lost it, or it was stolen from them -- a strange fact, considering that their collection is so well-protected. But they were mindful of it, careful with it, locked it up; it did not like that. It wants to be read.
It surfaces from time to time. Rumor has it it's in Strange, but not necessarily. The Strange Library might have it, but then again not.
It acts as if it were sentient. So it might well be a mask.