The Book of Omens was a book created by the clerics that contains information pertaining to the history of the ThunderCats, and clues to their future.
Long ago when being attacked by evil, the Thundercats, who thought that the Book of Omens was more valuable than the Sword of Omens, had the clerics hide it far beyond the walls of Thundera, far beyond any cat had been, in the Tower of Omens. The clerics-men, who hide it, trapped themselves within the tower so the location of the book was never revealed. It was found many years later by Lion-O and his comrades, though not before almost falling into enemy hands, that of Mumm-Ra, the Ever living. But the Book of Omens didn't come into the heroes' hands without a catch. When opened, there were no words inside it. The book is neither technology nor magic, but a combination of the two great powers. One can travel in spirit into the book and experience the past.