When I first read The Silmarillion as a kid, I didn't understand what it was and I didn't understand what it was trying to say or do. But I recently decided that reading Tolkien was a little like collecting lore achievements in a video game. The main quest is to understand The Hobbit and The Lord of the Rings, but you get bonus points for picking up and following up on the little hints of background lore Tolkien drops throughout. Sounds like a fun game, so I've re-read The Silmarillion as an adult, and while it's not a book I can earnestly recommend to others, it's an important book to Tolkien's setting and it's a book I think I've finally learned to appreciate.
I don't know how it was marketed or, aside from what I can read on the Internet, how it was received by the public, but it's fair to say that even today The Silmarillion confounds many Tolkien fans. To be honest, when I tried to re-read the book as a teenager, it put me off Tolkien altogether. After slogging through it, I was convinced that Tolkien was a one-story author, and that he'd written everything he was capable of writing in The Hobbit and The Lord of the Rings. The fact that The Silmarillion was published posthumously made it worse, and for a long time I felt that most Tolkien lore was "cheating" because it drew on content that Tolkien himself had never declared official. I've read Tolkien's letters since then, and I like to think I understand, well, everything better now than I did as a teenager, but I do think The Silmarillion can be a high-risk book for an unsuspecting or unprepared Tolkien fan. If you pick it up expecting even The Lord of the Rings at its most obtuse, then you've already underestimated it.
The hard truth about Tolkien is that he only wrote one The Hobbit and one The Lord of the Rings. Sure, there's other stuff written by him, but he didn't write more in that voice.
The Silmarillion is difficult to get through. I listened to an audio book while painting miniatures for Middle-earth Strategy Battle Game wargames, and even then my mind sometimes wandered.
The "characters" in the book aren't exactly characters, they're just a collection of names. They don't do anything, and instead they're described as having done something by an unknown and dispassionate narrator. There's never a sense of the present in the book. It's the vaguest of histories of things that happened to some people at some time in some place. You never believe that characters feel emotions because you're only told about what they feel in the most limited-omniscient voice possible. You never feel emotions about the story yourself because you don't care about these names, and even when you do, you already know how things turn out, because you've read The Lord of the Rings.
The Silmarillion is a recipe for a fantasy world, it's not a fantasy story. Or, more likely, it's the outline for a bunch of stories. It's tantalizingly easy to imagine a modern, capable publishing group like the Black Library taking each kernel of history and developing it into a dedicated trilogy each. Imagine Tolkien's setting getting the same treatment as the 100% already-spoiled, and yet 100% agonizingly gripping, Horus Heresy mythos from Warhammer.
(Granted, they've recently tried, and it didn't quite work out for The Rings of Power series. But I'm imagining an alternate reality where additional LOTR fiction is done well.)
As you read it, you'll likely sometimes start to foster excitement about Middle Earth, but The Silmarillion does a really good job of ensuring you never succumb to it. Its language is intentionally archaic, like the worst parts of The Lord of the Rings and, unlike The Hobbit it's entirely devoid of any personal connection between the author and the reader. I know Tolkien was working under the auspices that the histories of Middle Earth had come to him from Bilbo and Frodo and Merry and other authors, but instead of reading like Tolkien relaying stories he'd heard from Bilbo, it reads more like it's meant to be the archaic source material itself. I cringe every time I read words and phrases like "wherefore" and "whither" and "ere" and "it came to pass" in The Lord of the Rings, but that's the default for The Silmarillion. It's like the book doesn't want you to engage with it.
This method acting is an admirable gimmick in theory, and it would make great source material for readable scrolls in a video game. I can imagine playing Shadows of Mordor and learning about the history of Middle Earth through scraps of parchment consisting of The Silmarillion, and it would feel natural and appropriate there because it's in-world.
The thing about The Silmarillion, it being a book containing stories notwithstanding, is that it's not a story book. It's a worldbuilding book meant to provide you with the same level of knowledge about the history of a setting that the characters in the novels would possess.
If you read The Silmarillion, then you understand the significance of Isildur and Galadriel and "The Necromancer" and Sauron when they come up in The Hobbit and The Lord of the Rings, or at least as much as your average citizen of Middle Earth would. For me, that's a really big deal. One of my favourite things about geeky fiction is the ability to already be familiar with the setting before the story even starts.
I've said before that even if Tolkien had found the time to write what he clearly had in mind for Middle Earth, I don't think he had the "language" he needed to create it. Or maybe he did, but the world didn't have the delivery mechanism to receive it. Imagine how good The Silmarillion would be as in-world content for a video game, or as player handouts for a tabletop RPG. It reads like historical documentation. It's immersive in the best way, but you only know that if you're prepared to play along. Tolkien's other books don't require that, though, and I guess a lot of books don't, so it can catch you off-guard. However, The Silmarillion is not alone in what it does. There are other books that ask you to assume an in-game identity while you read.
Lord Dunsany published Gods of Pegana in 1905. It's a brash and bold description of a pantheon of gods with no context, and it's brilliant. If you didn't know it was intentional fantasy when you read it, I think you'd probably imagine Dunsany was either an actual religious leader or else mentally ill. Like The Silmarillion, it's not a storybook but a book of stories that appears to expect you to pretend that it's part of a sort of expanded universe to our own reality. There are no protagonists, no antagonists, no plot, and not all that many pages. It's a short book because it lacks detail, and that's partly why it works as well as it does.
The Silmarillion more or less achieves the same thing as Gods of Pegana, except there's a lot more of it. It's possible that a much-abbreviated Silmarillion would be a really good read, with lots of mystery around the terminology and names and events. But as written and edited, The Silmarillion is a long book with lots of words and lots of detail, so reading it is a different investment compared to reading Gods of Pegana, but ultimately it's just a matter of volume. With The Silmarillion, if you're in for a penny, you're in for a pound.
Either way, you're reading "scripture" that doesn't likely figure significantly into any story you'll read later on, but that could be an influence on worlds and characters if you know to look for it. It doesn't matter that Ilúvatar created the demigod Ainur, the Valar and Maiar, but if you know about it then Gandalf's famous Balrog speech has some hidden messages for you to pick up on. Likewise, when you read about Cthulhu in Lovecraft's universe, there's an added dimension to it if you know about MĀNA-YOOD-SUSHAĪ and its pantheon.
Robert E. Howards's Hyborian Age describes the setting of the world of Conan the barbarian. Like Tolkien, Howard wasn't very specific about what planet Conan existed on, so I've always assumed it was an alternate history of Earth. Frankly, I've always thought the same about Middle Earth (I know, Arda and Eä and so on, but it still feels like alternate Earth history to me). In the end, it doesn't matter what universe (parallel or otherwise) or what planet (alternate or otherwise) these stories use as their setting, because traditional fantasy concerns worlds that are no more.
Unlike Gods of Pegana, The Hyborian Age is broad and filled with lots of specifics that you're likely to forget even after repeated reading. I read the book for Librivox, and I still can't remember all the historical events in it. Similarly, I can't keep the historical events of Tolkien's world straight. There's not much story to The Silmarillion, and yet there's an abundance of detail. The book lists names and lineages and languages and regions and roles and times and locations. It doesn't take a break to tell you why you might care about the things it's listing, it just lists as much as it possibly can.
The advantage to this is that Tolkien had plenty to draw upon when writing throw-away lines in his actual stories. The Shelob chapter of Return of the King casually mentions that Shelob's story is much bigger than what's told in the book, so it's exciting when you get to the part about an evil spider in The Silmarillion. Casual mentions about Númenor and the Dúnedain in LOTR have weight after The Silmarillion. It re-frames a lot without changing anything. You just gain a new perspective on something you've been enjoying for years or decades or your whole life..
As with Howard's Conan stories, it's because there's an abundance of history to Hyboria or Middle Earth that you believe the "real" stories. You know it's not just for show when an author like Tolkien threatens you with a long story about a gemstone or a sword or a helm. There's authenticity to Middle Earth because there's authentically a vast backstory waiting for you to discover it.
I'm convinced that if Tolkien were writing today, he'd be a developer for a roleplaying game, and he'd be frightfully good at it. How good? I'm talking Tracy Hickman and Margaret Weiss and Ed Greenwood good. In fact, I'd love to see a version of The Silmarillion as an RPG sourcebook, with lots of illustrations and side bars and notes on how to roleplay a specific character or character class, or how an environment effects what items are available at a specific location, and so on.
Tolkien didn't have the luxury of scribbling that kind of nonsense on the page and having it get published before the ink was dry, because nobody was publishing travel brochures for make-believe worlds back when he was alive. It's too bad, because these days that's exactly the kind of lore people want to read while planning their character build.
In the Dragon Age and Kingdoms of Amalur) video games, one of my favourite things to do is to find the books of lore about the worlds. I read everything I find, and it always pays off, at least emotionally. I feel closer to the world, closer to my character and other characters in the game.
I don't think The Silmarillion is a perfect book, in the sense that I don't think anybody would argue that it's the book Tolkien wanted to write. It's a book of stories and ideas that, in a perfect [real] world would get [good!] novels all their own. I'm not saying The Silmarillion is necessarily a good book at all, and I'm definitely not saying it's a good book just because it's by Tolkien. But The Silmarillion is what was left to us, and I'd rather have The Silmarillion out in the public, preserved and available for anyone interested in the specifics of Tolkien's fantasy world, than for it to have been neglected and forgotten and lost.
If you want to read The Silmarillion, then be prepared for some arduous study. If you don't feel like reading it, I don't think you're missing much. You can continue to enjoy Tolkien's works in much the same way your average hobbit would. You'll hear stories of fantastic past events, you'll hear mysterious elven words, and a few dark words that you'll not dare repeat. Either way, you won't know exactly what they mean, and all that means is that you'll just have to wait for a wizard to wander by to explain it to you.