DrForester said:
Stolen from ponygoons.
http://i.imgur.com/hHvZz.png
It's not even the same book they read at the Library.
http://i.imgur.com/ktQRW.jpg
Twilight, on the grass in Canterlot, is reading the unnamed Old Book 1 (written by Celestia, as evidenced by the voice-over, and given to Twilight to begin a Batman gambit). It tells the legend of Celestia and Luna/Nightmare Moon, and how Nightmare Moon was defeated using the Elements of Harmony.
Twilight remembers reading about the Elements before, so she rushes to her Canterlot library to find her old copy of "Predictions and Prophecies" (Old Book 2). She finds the Elements in the index, where they link to the Mare in the Moon prophecy. The prophecy says that Nightmare Moon (having been defeated by the Elements) will return and destroy the world (never happened, stage two Batman gambit).
After Celestia ignores Twilight and sends her off to Ponyville, and Nightmare Moon returns (in Ponyville), Twilight runs to the Ponyville library hoping to find some info about the Elements. Pinkie Pie finds "Elements of Harmony: A Reference Guide" (Old Book 3). It has some incomplete information about the Elements, and says that the Elements were left lying on the floor of Celestia/Luna's former (now abandoned) shared home.
Mane cast saves the day with rainbow space gun. Celestia takes the Elements back for "safe keeping". Discord steals them.
After wrongly guessing that the stolen Elements were in the palace maze in Canterlot, Twilight is pushed by Discord to guess again, and she guesses that they're located somewhere in Ponyville. After she gets back to her Ponyville library, she has a book that she "just knows" will give her a clue to where Discord hid them (wrong again, unless it's a magic book that tracks things, or maybe Discord wrote an update for it).
In the library, Twilight says that she wants the Reference Guide to the Elements of Harmony. Spike says he knows exactly where that is, and appears to grab Old Book 1 instead of Old Book 3, from a location that isn't where Pinkie Pie originally found Old Book 3 (which was under "E").
Any clue from Old Book 3 is unnecessary, because the Elements are inside Old Book 1, which has been hollowed out by that book-vandal Discord. Old Book 1 fits Discord's riddle, because it's where Twilight began running the Batman gambit which put the Elements in the mane cast's possession. Old Book 2 might better fit the riddle, since that's the first place Twilight heard mention of the Elements. Old Book 3 doesn't really fit the riddle.
I think it stands to reason that Twilight would have Old Book 1 in her Ponyville library, since she probably owns it as a gift from Celestia, and at the very least she can transfer books from one library to another. Also, Spike is incompetent enough to bring her the wrong book, since he can't read large words, and probably arranges Twilight's libraries using things like color and smell and memory.