The Book of the Law

The Book of the Law is a grimoire written by Aleister Crowley back when he was the most renowned magician of the Magic Side. It's undecipherable, not because of the difficulty to come up with a coherent reading, but because there are hundreds of possible ways to read the grimoire; on its cover and apart from the title are the words "Liber AL vel Legis", which roughly means "It will be as you wish it", emphasizing this fact. Index herself cannot unlock its secrets and can only memorize the words without knowing its meanings.

The Book of the Law, along with Ursula Aquinas, is the main reason behind the conflict in Volume 7.

(Canon) Theories on The Book of the Law

 * Anyone that can unlock its secrets will obtain power on par with that of an Angel.
 * If it's deciphered, it will mean the end of the age of the world that's controlled by Christianity, and the beginning of a new one.
 * Crowley used the book to predict the future and as a guide when he was in dire situations. This grimoire is said to have decided the recent history of Western Magic.

Contents of The Book of the Law
Apart from the various theories, according to Index, what the Book of the Law holds are events that happened to Aiwass. Though it's not known who Aiwass is, theories on that range from it being Crowley's guardian angel to it being a criminal. As of Volume 19, it's confirmed that Aiwass is Crowley's guardian angel of sorts.