Lulu:

Stories from Georg, Bad Horse, GhostOfHeraclitus, Estee, and Skywriter, about a collection of female alicorns, related by their unique placement in the universe, who do not so much suffer from insanity as rather enjoy it. For centuries, there was only one of them in Equestria, then two, and three, then four and five… Together, they share a unique relationship, closer than mere circumstances would otherwise indicate.
Against the multitude of ponykind, they are singularly unique, thinking of each other as both competitors and allies against the world's stress, conflict, time travel, released monsters, accidents of history, cosmic coincidences, foalbirth, and other such calamities. In short, they consider each other as… Sisters!

[Paperback]

The book also contains:

• The stories Socks: The story behind the fashion craze! and Betting the Moon which are chapters of the story Sisters.
• The story Carbon, Hydrogen, Oxygen, Nitrogen which is a chapter of the story Never The Final Word (Vol. 1)
• The stories Princess Celestia’s Most Memorable Birthday Present, Cutting the Chord, Swinging the Hammer, and Dropping the Piano and Princess Twilight Sparkle - Mistress of the Night which are chapters of the story EqD Writer Training Grounds short stories by Georg