Christopher Tolkien published The Silmarillion after the death of his father, a book created from the living body of work J. R. R. Tolkien created in his notebooks encompassing the mythology, legends and annals of Middle Earth history.
Ostensibly, The Silmarillion is made up of five texts, Ainulindalë, Valaquenta, Quenta Silmarillion, Akallabêth, and Of the Rings of Power and the Third Age. Some will be familiar through direct mention or vague reference from The Hobbit and The Lord of the Rings. Other pieces of lore will require extrapolation to connect to the world we've been reading through for two years.
I will primarily be reading from my library's copy of The Silmarillion, illustrated by Ted Nasmith. I own a paperback copy of the text, and I will likely dig through it as library loans require I relinquish the borrowed copy now and again. My paperback copy also has notes penciled in the margins from my first read through over a decade ago. So, onwards!