It's kind of a silly thing, but there was just a study that says we really do get different things out of a book the second (or more) times we read it.
The first time we read a plot, we're concentrating on the plot. The second time, we know the plot, and thus have 'time' to focus on the characters.
To readers, this says that it's worth reading a book again.
What does it say to writers? How do we 'layer' our books so that re-reading is more enjoyable, or is this not something we need to worry about? Is it automatic if all of the elements are in place, or do we need to think about it?