In some earlier versions of the Lenormand deck (as well as other fortune-telling systems it appears in), the Book was often depicted as a grimoire, a book of shadows, a collection of magic spells and incantations used for bringing about change outside of the realm of the laws of nature. This is why, even now, this card stands for deeply hidden secrets, for information that is not readily available but instead concealed from sight. Here, directionality comes into play and what is to the left of the book is likely to be hidden for a long time to come, while what is to the right (perceived as being in front) will be revealed soon.
It’s secondary meaning however is a lot more down-to-earth and the card can point towards an actual book – be it a textbook, a notebook (with the Broom serving as the pen), an encyclopedia, or a novel. It symbolizes knowledge and learning, official education and academic achievement and, when next to the Tower, can stand for an institution of higher learning. Perhaps that is why it warns us that something we wish for will be slow to happen.