Beginnings

The World Ash Tree, Yggdrasil, is like a changeless Absolute despite the dragon Nidhogg gnawing at its deepest roots. The Tree survives the Ragnarok although it is not mentioned in the stories of the Ginnungagap. Mythology is neither logic nor science. (Thor sleeps in one finger of a giant's glove but later wrestles with a giant.) The Tree comes to an end in Wagner's Ring operas but that is a different version of the story.

Although most accounts of Norse myths begin with the Ginnungagap, Poul Anderson's War Of The Gods begins with:

"...the nine worlds in the Tree." (I, p. 9)

- although it also mentions the slaying of Ymir.

The text lists Asgard, Vanaheim, men, elves, dwarves and jotuns/thursirs: six, not nine. The Eddas present no definitive list. See Norse Cosmology: Nine Worlds. 

In just over six pages, pp. 9-15, Chapter I of War Of The Gods summarizes several Norse myths as a build-up to an innovative action by Odin. In the just under three pages, pp. 295-298, of the concluding Chapter XXXV, that action, an intervention in human history, has been completed. These chapters book-end Anderson's retelling of the story of Hadding.