Appendices ahead of time. Of the three volumes, the first is the largest by far. The last is also large, but there are a lot of pages in the appendices; The Return is about the same size as The Towers.
As a closer, Tolkien crushed it with The Return of the King. This is especially obvious when you read about how he got there (which is what I'm reading right now.) There are so many other ideas he had as he wrote this novel, and while I think many think he took too long to write it, the slow thoughtful process is what resulted in the wonderful story we have now.*
While The Two Towers followed two different story tracks in two separate books, The Return brings the storyline (and the fellowship) back together in a lot of ways. Each member of the fellowship also experiences growth as the story progresses. It would have been easy to have supporting characters that act as straight men, or foils to help the author answer questions the readers may have, or to simply add depth (or entourage) to the story, but Tolkien didn't do that, there are no wasted characters here.**
The ring is a bad thing, its pretty clear. What to do with it is the central theme of the book. What do we do with evil when we encounter it? Bury it, overcome it, or destroy it? Each of these options is examined, but also examined are the consequences of each, including the unintended consequences. What if--this book asks--evil things end up spawning wonderful things, should those things be destroyed with the evil if uncoupling them proves impossible?
That's a hard question. And its ultimately what differentiates The Lord of the Rings from other books in this genre. In every era of Middle-earth, since its making, there has been an evil, brought from without, and balanced by a power for good, set firmly against it. At the end of the third era, when that final bit of the original seed of evil is finally rooted out, so also ends the power that was set against it. Which leaves men alone; inheritors of the fourth age, and all the rest of time, with nothing but mythologies, and the intangible but lasting effects of that original evil, to remind us.
Its not just the sorrow of the elves that pangs, its our own as well.
Read this book.
* There are those who think--and I was one of them years ago--that if Tolkien had written quicker, he would have gotten to more of the stories that he contemplated writing. I read somewhere that Tolkien considered The Lord of the Rings the 'end' of the story, essentially, and there was also a beginning story and a middle story (or stories) that could be told.† Many of these stories were ultimately released in The Silmarillion, and various other books Christopher Tolkien edited and published, such as Unfinished Tales, but if we had his father for longer, perhaps he would have completed these stories himself. I guess that's true, but I don't wish that he'd hurried through the LOTR. If he had, we wouldn't love it the way we do, and perhaps wouldn't care if he'd had the chance to finish the other stories he thought about, if they weren't going to be as good as this book ultimately is.
**Those of you that want to argue about Bombadil can do it outside, before I turn the hose on you. I'm talking about the members of the fellowship, who all have a roll to play, big or small. And each character is fully formed. And no I'm not sure how both he and Treebeard can both be first and oldest. Maybe one's first and the other is oldest; does that fix it?
† Please don't. Peter Jackson, I'm looking at you.