Second place, Midwest Brony Fest 2016
This version of Twilight Sparkle is based on Amelie Belcher's fanart, which was published before Equestria Girls came out. The costume debuted at Naka-Kon 2015.
You can't see the best part of this costume in photos - because it actually sparkles! Using an Arduino microcomputer, an accelerometer, and a couple dozen LEDs, the skirt twinkles whenever I move. I used conductive thread to stitch all the LEDs together on one of the skirt's underlayers. The microcomputer and 9V battery pack are located in a hidden pocket.
See the skirt in action here.
The book is one of my favorite props. I bought a book box from Michael's, sanded it down, and painted it to look like Star Swirl the Bearded's book. It's hollow on the inside, and doubles as my purse. The shirt was store-bought, but I dyed it to Twilight's particular shade of purple.
Reference photo via Amelie Belcher's DeviantArt page.