Reading Brandon Sanderson's Stormlight Archive series, and 6,000 (digital) pages in, 30% into book 3, the prologue to book 1 is finally given context.
I really love when the groundwork is laid early, and you get a good payoff way down the line. When I finally got the context in book 3, the events from the prologue were just whispers of a faded dream to me, much as those same events were to the characters 4 millennia later.