Hachette has another book coming in the Equestria Girls lineup. This one doesn't have much information on it other than the release date and length at 208 pages, but it does mean we are going to continue seeing future EG Content after the three shorts releasing later this year.
It has a pretty generic description:
Join Sunset Shimmer, Twilight Sparkle, Pinkie Pie, Rainbow Dash, Rarity, Apple Jack, and Fluttershy as they rely on their special friendships to guide them through the highs and lows of being students at Canterlot High.
Thanks to Brony250 for the heads up!