The Return of the King
Treebeard the Ent appears one last time in The Return of the King as Gandalf and Co. travel from Rohan towards Rivendell. Treebeard and his fellow Ents have been patrolling the ruins of Isengard, where, up until a week before, Saruman had been kept locked up in Orthanc.
Treebeard explains that he has let Saruman and Wormtongue go. Saruman seemed so pathetic that Treebeard could not see the harm in releasing him. But Gandalf thinks something quite different went down. He guesses that Saruman used the manipulative power of his voice one last time to convince Treebeard to let him go. Still, Gandalf does not blame Treebeard for his pity, since it's a sign of the goodness of Treebeard's character.
Treebeard also gets a last glimpse of the High Elves, Celeborn and Galadriel. Galadriel admits that the Ents will never see the High Elves again unless it's at the end of time. So the tree-herders that the elves woke so long ago will never again meet their makers. Again, the melancholy tone of The Return of the King strikes us. Treebeard's last glimpse of the High Elves reminds us that so many nice things are going to pass away by the end of this novel.