One Book of Rularuu
As the name of the item implies, it is a book held by Rularuu The Ravager which supposedly contains every piece of information about everything in the Omniverse from the start of all time until the final end. Every event in every reality from start to finish.
The book is semi-sentient, allowing the owner to access events on demand instead of having to go through a seemingly infinite number of pages to find the right event in the right universe.
However, despite the name, there are several versions of the One Book that exist. It is suggested that these alternate versions were later crafted by Rularuu as a way to properly dissect the information. Another theory is that these other versions of the One Book exist as a way to distract people from finding the One True Book.
The One True Book
There was, at one point, just the one "book". This book was crafted in First-Speak, the first language of the universe, which is used only by the most ancient of items such as the Pillar of Ice and Flame in Ouroboros. It would only respond to those who can speak that language and not through any kind of translation device.
The One Book
According to Portal Corporation, there are numerous items referred to as "The One Book", which look like and respond similar to the "One True Book", except these versions have a few distinct differences.
For starters, the "One Book" is crafted in Inter-Speak instead of First-Speak and responds in the same. Second, the "One Book" is not a complete version, having only limited information about certain realities. Some versions would be limited to the events of only one universe, others would be have limited information about any given universe. Some versions of the "One Book" have a safeguard in it that prevents the user from knowing their own future. Other versions will not update their information in the event of changes in time so they would be inaccurate.
(See the item's [Paragon Wiki entry] for more information.)
The One True Book was seen at the end of "Guardians of the Dawn GRADUATION" #3.