This book is a compilation of stories. One of them tells us how Arda (the world) was created and how the races were born. Other story is about the Silmarils, precious stones that will be crucials in Middle Earth’s First Age. Another, explains us the downfall of Númenor and its people during the Second Age. There are other ones about Tolkien’s mythology.
I think this is a difficult book to read because it’s extremely descriptive. You should read it if you want to know more about Tolkien’s mythology and the world before The Hobbit and The Lord of the Rings.