The Kingkiller Chronicle. Firstly, they're good books and are enjoyable to read a second time round. Secondly, Rothfuss has laid down plot points like a Go player places stones. The second time reading through will join together disparate pieces of information that you wouldn't have connected together before, starting from very large revelations in the second reading of the first few chapters. I know of other books that are so rich that you can appreciate new details on multiple readings, but none that re-contextualise the whole story with revelations hidden in plain sight. Readings: (Name of the wind - 4 times, Wise Man's Fear 5 or 6 times - I like this one more for some reason).
Also recommended in this reply