Shippey says that, in
The Lord of the Rings, Tolkien used an old literary technique called 'interlacing', that is, the weaving together of plot elements. Today we see this technique used most effectively. We learn of these several events by reading five different chapters in
LOTR, spaced through the two books of
The Two Towers: 'The Riders of Rohan', 'The Uruk-hai', and 'Treebeard' in the third book, and 'The Taming of Smeagol' and 'The Window on the West' in Book Four. That's a span of fifteen chapters. Why is this so effective? Because we readers experience these events somewhat as the Fellowship were likely to have experienced them, disjointed, not knowing the big picture, feeling the same anxiety for the other characters. Frodo didn't know that Merry and Pippin were the prisoners of the Orcs. No one could have predicted the meeting with Treebeard or that Gollum would turn into a companion and guide. Eomer shows up out of nowhere. (And we won't even mention the possibility of Boromir's funeral boat arriving intact at Osgiliath, born by the mighty Anduin!)
Yet the interlacing of these events creates the story. Tolkien's ability to devise these timelines, tell us most of what we need to know, reveal the story in layers, and have his reader experience the story in time, rather than in a linear fashion--well, shall we call it masterful?