The Book of Yggdrasil is an ancient grimoire written by the Asgardians, detailing much of the history of the Nine Realms. Many of the events described in the book are used as children's stories by Asgardian parents.
The Book of Yggdrasil is one of the oldest texts in Asgard, detailing ancient knowledge that predates even Odin. Through some form of Asgardian enchantment, the illustrations in the book seem to move on their own accord.
During the time of the Convergence, Odin used the book to explain the nature of the Aether and the history of the Dark Elves to Thor and Jane Foster. When Foster, who was hosting the Aether, asked if the book said anything about removing the Aether from her, Odin regrettably replied that there was not.