In The Return of the King, Tolkien goes to a lot of trouble to make his readers really feel how awful the experiences of Aragorn approaching the Black Gate or Frodo and Sam coming up on Mount Doom must truly be. He does this by stopping the plot periodically and filling in wordy, descriptive passages of what it's really like to be going through what these characters are going through. Take this (frankly disgusting) description of the countryside in Mordor:
[Frodo and Sam] scrambled down to [the river-bed], and began to cross it. To their surprise they came upon dark pools fed by threads of water trickling down from some source higher up the valley. Upon its outer marges under the westward mountains Mordor was a dying land, but it was not yet dead. And here things still grew, harsh, twisted, bitter, struggling for life. In the glens of the Morgai on the other side of the valley low scrubby trees lurked and clung, coarse grey grass-tussocks fought with the stones, and withered mosses crawled on them; and everywhere great writhing, tangled brambles sprawled. (6.2.44)
The landscape of Mordor becomes another way for Tolkien to portray the misery of Frodo's current state of mind. Even the land's "low scrubby trees" and the "coarse grey grass-tussocks" are crawling over and fighting with one another for an inch of space and nutrients to survive. The landscape is battling with itself, much as Frodo has to keep struggling constantly with his own darker inclinations.
The wordiness of these passages makes the reader stop and think about the feeling of the language Tolkien uses, and not just the content of what he's saying. Even though this is a fast-paced and plot-driven book, The Return of the King is also clearly concerned with the effect its language is having on the reader.
We also choose to call Tolkien's style circular because there are a lot of elements of the earlier books to be found in The Return of the King. Not only does Book Six, Chapter Six's title ("Many Partings") echo the name of Book Two, Chapter One ("Many Meetings"), but plot elements, including obscure ones like Saruman's Longbottom Leaf in The Two Towers (suggesting Saruman's ties to Bree and the Shire long before "The Scouring of the Shire"), return in Tolkien's final volume.
The circularity of Tolkien's plotting—that he begins and ends the trilogy in the Shire, that he arranges a last meeting between Treebeard, Galadriel, and Celeborn (since Treebeard mentions Celeborn in The Two Towers), etc.—reminds us of how careful Tolkien is to maintain the continuity of his three novels.
While these may technically be separate volumes, The Return of the King is clearly part of the same overall work as The Fellowship of the Ring or The Two Towers. This isn't like an episode of a bad TV series where the writer has forgotten a character's backstory from one show to the next. Tolkien makes sure to tie up his loose ends by the conclusion of The Return of the King, which creates a whole experience for his readers that is beyond compare.