This has been weighing heavily on my mind, but I have always believed that Twilight's possessions that made her who she was (Friendship Reports, Dresses, Starswirl Costume, Books from "Trade Ya") somehow survived because they were in either her basement or in different locations.
For example:
. her dresses from the Grand Galloping Gala, the Royal Wedding and her Princess Coronation were either in storage at Carousel Boutique or in Canterlot. I also never saw a closet in Golden Oak Library, so its likely Twilight's special occasion clothes are safe.
Speaking of those dresses, I would like to see her wear them again in certain episodes of Season 5.
.her winter wrap up vest and Starswirl the Bearded Costume was down in the Golden Oak Basement with her laboratory equipment.
.the books she mentioned in "Trade Ya," could have been at the Castle of the Two Sisters along with other books that meant great value to her.
.the former necklaces and crowns that bore the Elements of Harmony were most likely in a museum in Canterlot.
.her friendship reports were eventually returned to Celestia and remained with her in Canterlot.
Most likely, everything that Tirek destroyed was eventually restored, both magic and structure. However, if Ponyville wanted to bring the library back, they would have to build it themselves. It would make for a really good episode in Season 5.
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