The Hobbit was Tolkien writing a story and adventure - indeed it was a challenge thrown to him by C.S. Lewis to actually write a story not a lore or mythology.

Lord of the Rings is half way - its in part mythology but also a story and adventure.

Silmarillion is notes and history and lore. Indeed the bulk of what comes after LotR is of a similar nature. Tolkien did world building on a grand scale that most authors never get close to achieving (and those who do generally do it bit by bit over a 20 book series). Indeed one can argue that Tolkien - for all that he also lifted much from Norse legends, was one of the first to take world building "seriously" in writing beyond publishing a story.

His intent was to create a mythology for England. To that end he created his own language (not just taking an existing one - he built it from the ground up); his own world and lore. There's such a vast body his son spent years sifting through notes and adding and mixing them into further stories.

My favorite Tolkien book is the Silmarillion. It gets right to the background stuff he wanted to write without a lot of the plodding filler of LOTR—I don’t find Silmarillion to feel slower than LOTR. It’s Codex: Middle Earth Elves.

 BobtheInquisitor wrote:
My favorite Tolkien book is the Silmarillion. It gets right to the background stuff he wanted to write without a lot of the plodding filler of LOTR—I don’t find Silmarillion to feel slower than LOTR. It’s Codex: Middle Earth Elves.

honestly, comparing it to a 40k codex is outright insulting.