One thing I rather like is when I'm rereading and I come across some inconsequential detail thrown in for background, that I notice because it relates to a future book. For example, I'm rereading
The Sharing Knife: Legacy, and just found a place where Shaun mentions to Dag what he heard happen in Log Hollow when a Lakewalker patroller took up with a farmer woman. In the next book, Dag happens to run into that Lakewalker. At the time I first read that, I hadn't read
Legacy recently, so I took it for granted Dag knew which camp the character was from -- stories like that get around. Now, I know that I could have as well.
Similarly, I was flipping through
Grave Peril by Jim Butcher and happened on the scene where Harry and Michael were trying to bargain with Leanansidhe, a Fae member of the Unseelie Court, who had Michael's sword. Lea already had something on Harry (she was his Fairy Godmother, and he owed her a favor.), and said if she just collected, she'd return the sword. Michael offered himself in her place, but she refused, saying that he was boring. She then asked for one of his children -- mentioning that she wouldn't mind taking his oldest. Michael told her to take a short walk off a long cliff. Some six books down the line, in
Proven Guilty, Michael's eldest child, Molly, is indeed kidnapped by the Unseelie Court and it's actually explained why they have an interest in her. (Not just because one of the things bad fairies do is still children.)
I don't always know if the author was planning on using it when they wrote it. Maybe sometimes they are, and sometimes they aren't, and just happen in on re-reading and think 'hey, I ought to pick that up'. It's still pretty cool.