Arguably the first book that can be qualified as a fantasy book (not fairy tales). The author influenced a lot of fantasy writers, so if you do not like something in the modern fantasy he is the guy to blame - and the other way around: he is the person to praise for the state of genre. The most direct influence of this book can be seen in [b:The Silmarillion|7332|The Silmarillion|J.R.R. Tolkien|http://d.gr-assets.com/books/1336502583s/7332.jpg|4733799], especially in the part of world creation.
So, the plot is fairly simple. A supergod (for the lack of better term) created gods and went to sleep. While he slept, the gods played around creating the world, land, seas, animals, people, etc. The book is a collection of stories of these creations.
The book is slightly outdated and fairly difficult to read with 3 solid star rating.