This book is a brief summary of the creation of Arda and the events that passed during the first, second, and third ages of Middle-earth.
The history of this world that Tolkien has built is wonderfully complex so The Silmarillion is a perfect starting point for anyone who wants to learn its history. Like I said, it is only a brief summary, as so many details are left out.
The narrative voice is omnipresent, giving the reader a view of the world as an onlooker which brings an almost biblical-feel to the book. I loved the narrative but I had to re-read sentences and paragraphs as I read due to the way in which things were said.
From the oldest and deadliest dark lord, Melkor, and his corruption of elves, men, and even one of the Maiar, to the evolution of Elves, Dwarves, and Men and their tragic and joyful tales, this book is a perfect start to the world Tolkien has built.
NB. I would suggest that readers read The Lord of the Rings and The Hobbit before attempting this book as to understand a lot of it, it would be easier to be familiar with a bit of the world, even if it is only the third age.