“The Silmarillion” is a VERY different book from what most fans of “Lord of the Rings” may expect. First of all, it’s an anthology of several different works J.R.R. Tolkien had started but had never published on their own. While they are separate works, they all deal with the origins of Middle-Earth and some of the important Elvish characters, mostly Elrond, Cirdan, and Galadriel. The second thing is that this isn’t a narrative driven text. It’s very much a tell-don’t-show style you would find if you were reading other mythologies. It appears that Tolkien did this intentionally in order to anchor Middle-Earth in the realm of other mythologies such as the Norse, Greco-Roman, and South Asian.
From an artistic point-of-view I appreciate what Tolkien accomplished with these pieces. He took the style of origin story and applied it to the world he had already created. It’s not an easy task, especially since you are doing this after you’ve already established the world. From a narrative stand-point it’s not always the easiest read. There’s the framework for a lot of good stories, but they never are really developed since this isn’t about the characters but about how the characters shape the world.
As part of the Tolkien-verse, I wouldn’t say this is the most important for new fans. “The Hobbit” and “Lord of the Rings” are much more important. HOWEVER, I do think that you have to have read this once if you’re going to call yourself a Tolkien fan.