The Book of Fate is kept by the Norn. In it, she records all the histories and destinies of mortals and gods alike. Like the Olympian Fates, the Norn catalogs the past, present and future of all mortals and gods in the Norseland. She paints what she sees in the Book of Fate, but even the Asgardian gods forget that the Norn only paints the present a few moments before it happens. Thus, in spite of its name, the Book of Fate can't actually be used to predict the future. The book can be used to literally rewrite history by repainting earlier scenes, though Hercules and the Norn herself are the only people who know this is possible.