I've said it in every other post about these books so far, but I will say it again: you should read The Lord of the Rings. If you haven't read them, then a) I'm very sorry for the sad life you have heretofore lived, and b) you should read them immediately. If you have read them, then you should reread them. These are the kinds of books that spoil you for everything else that isn't nearly half as good as they are.
At the end of The Two Towers, if you're not familiar with the plot already, you'd probably believe that this story is not going to end well. Of course, the biggest spoiler of this book is its own title, which Tolkien did not pick. His original choice for the title of the third part of The Lord of the Rings was The War of the Ring, but he was persuaded to change it to the more positive, and arguably more descriptive, The Return of the King. Knowing this history, I'm not entirely certain which title I prefer. However, I am entirely certain that I enjoyed this part of the story just as much as the others.
There is a fair consensus amongst those who come to consensuses about such matters that Tolkien wrote The Lord of the Rings primarily as a way of exploring Middle Earth - that is, this is what is known as a milieu story, in which the setting, the world, drive much of the plot. In The Two Towers this is on fine display again. One of the more interesting things to do with a copy of The Lord of the Rings is to sit down and look at just how much ground is covered by the various journeys; you then realize just how large a world Middle Earth is, and how small a section is explored in these tales. The distance covered by Frodo and Sam through such great peril and difficulty in the entirety of their chapters in The Two Towers is essentially a tiny corner on the map.
As I said in my review of The Hobbit, during this reread I was surprised by how light that novel is; I suspect that my memory of its tone from my last reading was affected by my intermediate viewing of the movies. Or, perhaps I was merely linking it with the core Lord of the Rings books, which very quickly take on a markedly different tone from their prequel (and yes, I know that technically there is just one "book," which was split into three parts for the convenience of readers and publishers). The implications of a darker turn are heavy throughout even the early chapters of The Fellowship of the Ring, but are pivotally confirmed with the events of the chapter A Knife in the Dark.