The Book of Breathings is a powerful magic item that is said to be able to nullify the Cauldron's magic and possibly contain a spell able to send Amren back to her home. After the war a 500 years ago, half of the Book was given to the Six Mortal Queens and the other was entrusted to the royal High Fae of the Summer Court . Though cautioned not to merge them together, at the end of A Court of Mist and Fury , Feyre combines both halves of the Book; the result is yet to be seen in A Court of Wings and Ruin .