The Two Towers
Analysis: What's Up With the Ending?
Tolkien was absolutely not a fan of using the word "trilogy" to refer to The Lord of the Rings: "The book is not of course a 'trilogy.' That and the titles of the volumes was a fudge thought necessary for publication, owing to length and cost" (source). For Tolkien, the idea that these books were a trilogy was just a cheap marketing strategy on the part of the publisher. So to talk about an "ending" to The Two Towers—when the book is clearly just a middle chapter in one, enormous, fifteen-hundred-page tale—would be crazy.
Still, even if Tolkien didn't like it, The Two Towers is a separate book within The Lord of the Rings series as we now know it. And there are significant cliffhangers that seem worth talking about at the end of the The Two Towers.
Ending at Isengard
The Two Towers is the only one of the three volumes of The Lord of the Rings that splits evenly into two sections, with no direct overlap between the two. So there are really two endings at work. The first is Pippin's departure from Isengard with Gandalf. The pair rushes to Gondor to prepare for the bigger battle against Sauron to come.
The gathering of troops in Rohan and the destruction of Isengard both foreshadow the destruction of Sauron's kingdom in The Return of the King. Plus, the fact that Pippin accompanies Gandalf to Minas Tirith proves that the hobbits will continue to play a big part in the next book of the series.
The second cliffhanger in The Two Towers is Frodo's abduction by the orcs of Cirith Ungol, leaving Sam to try to think of a way to free his beloved master. Over the course of The Two Towers, as Frodo has gotten more and more distracted by the Ring, Sam has become an increasingly important character in the Ring quest. In fact, Sam starts developing into the leader of their two-man expedition, because he has got such a good head on his shoulders.
By the end of Book 4 in The Two Towers, while Frodo lies locked up in a tower by orcs, it's up to Sam to stand alone and decide what to do to rescue his pal. Sam is about to come into his own as a character, which is yet another reason to keep right on reading. You'll find out how that turns out The Return of the King.