The booking glass is an ancient artifact which used to be under Snow White's possession. It has the power to capture anyone or anything inside it, by conjuring a spell. It is currently shattered.
According to Baba Yaga's History of Evil Spells class, it is learned that the booking glass was shielded by Dark Fairy magic, once in the mirror's lifetime.
Under Snow White's supervision, the booking glass was untouched until the Evil Queen was broken free from the Mirror Realm, and Snow White kept it for prevention, after accepting the challenge to reintroduce the Dragon Games by the Evil Queen, in case the latter decided to step outside her bounds, and also to raise her popularity levels once she'd capture the Queen again, in an act of heroism. However, the Evil Queen came to know of this mirror and ordered her admirer, Faybelle Thorn, to fetch it, which she did, only she accidentally switched the booking glass with a plain hand mirror, leaving the real one under Daring Charming's possession. When the Evil Queen took over the school, Snow White saw an opportunity to use the booking glass, however it was replaced by a regular mirror, and the Evil Queen, who thought she had the real one instead, shattered it and was able to successfully take over Ever After.
What she was not expecting was to, Daring Charming to mysteriously disappear, after an inconvenient miswording, into the Mirror Realm, and the booking glass to be found by his sister Darling Charming, who was later, after the Evil Queen's strike, able to free him, and, with the help of Apple White, daughter of Snow White, Raven Queen, daughter of the Evil Queen, and Faybelle Thorn, capture her back into the mirror, in a dragon riding fight, only to drop it from the sky into Ever After, shattering it and returning the grace to Ever After High.
The booking glass is a hand mirror with a golden frame adorned with red gems, that possesses magical properties. Essentially, the booking glass is a pocket dimension into the Mirror Realm, and can only be unlocked by a dark fairy, like Faybelle. Once activated, the words "Capture X", with X being substituted by whatever it is that is to be captured, must be said for the capturing ritual to occur, which is essentially a flash of light that sucks the person or item into the mirror. Whatever is inside cannot be seen from the outside. Additionally, there is a recorded voice that will inform the user once it has captured something or when it is locked.