The Book of Lost Tales 1 The History of Middle-earth 1

This book is the first part in a series of 12 books and contains Tolkien's rejected writings and notes which were later refined into the Silmarillion. It covers the first part of the Silmarillion; the creation of the world, the gods, the awakening of the elves and their story until the Exile of the Noldoli. For me the book is less something to read in its entirety, and more a work to reference when I want to know a particular thing since it does contain many aspects that are (likely) still true in the canon timeline, but were cut because it was too cumbersome to include everything into the Silmarillion.