The Tailed Encyclopedia is a unique book that was created and enhanced utilizing powerful sealing and sensing jutsu, even though what it does is not very impressive. There is a chapter for every tailed beast, which contains a picture of the beast as well as some information about the beast's statistics and demeanor. These portions have been filled in by previous holders of the book, and it has enough extra space and pages for more notes to be added. In this way, it really does act as an encyclopedia full of knowledge regarding the beasts.
It has a rather unique and peculiar ability, which is why it is created with special jutsu. When someone who holds the Tailed Encyclopedia stands within 50 feet of a tailed beast or Jinchūriki, the cover of the book begins to glow faintly. When it does this, flipping through the pages would reveal that the corresponding pages of whatever beast the user is near will glow. It is unsure exactly how the book senses the beasts, but it seems to respond even if the beast/Jinchūriki is suppressing their chakra or are hiding their presence with a space-time jutsu.
Another special ability is that the book is able to 'absorb' chakra from tailed beasts when it is around them. The absorbed chakra allows the book to create a shadow of a tailed beast which can then be filled with the reader's chakra. This is slightly similar to the division of a shadow clone, as the 'shadow' beast does not have the chakra or strength of the real thing and only is as powerful as the amount of chakra is placed into it. This can allow the reader to fight the 'shadow' tailed beast, but it will immediately dissipate when someone other than the reader attempts to fight it. This means that it can only be used as a training device and can only be used by multiple people if multiple people fill the tailed beast with their own chakra. After the Tailed Beast is either defeated or dispelled, the tailed beast chakra will need to be reabsorbed before it can be created again. It can store the chakra of multiple beasts at a time, but not more than one of each beast at a time.