So I'm reading The Silmarillion now, for the first time since I was in junior high; this is the one I've never gone back to, no matter how many times I read LOTR. And immediately I find it rewarding, because the first two sections, the "Ainulindalë" and the "Valaquenta," are so different.
One of the tensions that drives LOTR to such aesthetic heights is that between fragmentation and completeness. Middle-earth (don't forget the hyphen) is a complete creation, with more language, history, mythology, culture, and geography than most of us know about the world we live in. Its completeness is part of its charm: at some point or another most readers, I think, are struck by the thought that they could just crawl into Middle-earth and live there (not to mention the desire we all feel to do so).
And yet for the most part this completeness is achieved - rather, I should say, it's communicated to the reader - by means of fragmentariness. LOTR never sets out the complete mythology of the elves to its readers; we get bits and pieces here and there in songs and poems and allusions by different characters, and these are so carefully arrayed, and so convincingly placed into the mouths of the characters, that they create the sense that they're parts of a larger and complete whole. Same with the languages, same with the histories of Gondor, etc. I'm convinced that it's this fragmentation - Tolkien's way of delivering to the reader shards of his vision, tucked into the pockets of his story-cloak - that makes his fictional world succeed.
The Appendices set a lot of this stuff out in more complete fashion, particularly the history of the kingdoms of men; but even if you read the Appendices (and I don't know how many readers do), by the time you read them, you've already bought into the reality of the world. Nobody starts with the Appendices. And, truth be told, the Appendices don't give you everything, not by a long shot. They mostly flesh out things that you could almost have deduced from the narrative.
I'm no expert on Romanticism with that capital R, but it strikes me that this fragmentation is part of Tolkien's inheritance from Romanticism. It's part and parcel of his Ossian-like pose, delivering survivals from a vanished antiquity. But as I say, I think it's narrative genius; and I think Tolkien on some level knew this, because he wrote Lord of the Rings rather than insist single-mindedly on completing The Silmarillion.
"Ainulindalë" and "Valaquenta" are what LOTR isn't, systematic and complete expositions of the mythology of the Elves. They're the Creation Story, the War in Heaven, the catalog of angels. And not only are they deadly dull reading (beautiful, though, both in conception and execution), but they feel a good deal less real than LOTR. When we encounter this stuff within the narrative of LOTR, we're experiencing it as the hobbits do, as hints of a vaster truth. When its laid before us like this, it looks, well, made-up.
But what a making-up. Suddenly Tolkien's project no longer feels Romantic to me, but Modernist. It's monumental: it's no longer the illusion of completeness, but the completeness itself. It's the fictional equivalent of, say, a Brancusi (how right Peter Jackson was to bring Brancusian elements into his representation of Gondorian art): Ideal, seamless, polished and smooth, entirely 20th century in its manner of evoking classical canons of beauty.
Modernism isn't something you tend to associate with Tolkien, who was at such pains to preserve, and to show himself preserving, pre-industrial ideals. I'm not up on the critical discourse on Tolkien, but it does begin to seem to me that if we don't think of him as a Modernist, we're missing something.