The book was published after Tolkien death, since he began writing it back into 1917 but failed to convince his publisher to publish it. If you love the Lord of the Rings saga this book is in my opinion fundamental since it explains a lot of things and facts that happened way before Bilbo and Frodo were born and from the pages you can feel the love Tolkien had for Arda, the world he created.

And this is, in fact, a collection of tales that starts from the beginning of time way to the defeat of Sauron, spanning thousands of years: it's a real mithos following the tales of the heroes of this lands and building up what it's really alike other mithologies (nordic, greek etc.). It's a masterpiece, but it's also pretty hard to follow: every person has multiple names in multiple language and, without the help of the glossary at the end, it would be easy to get lost. A must for every Tolkien reader and, moreover, a kind of uniqum in its genre.