Basic RolePlaying is a generic game system. As such, it has a variety of different kinds of supernatural powers, magic, sorcery, mutations, super powers and psychic abilities. They are designed with the idea that multiple types will be used together. For example, a futuristic sci-fi campaign might include both mutations and psychic abilities, while a "kitchen-sink" style campaign might throw everything in there. For a pure fantasy or sword-and-sorcery campaign, however, the magic system is a bit sparse and bland. Fortunately, Chaosium has access to the rich history of RuneQuest and has released The Magic Book, a revised and updated version of the RuneQuest 3rd Ed. magic system for BRP.
The Magic Book contains four integrated magic systems, Spirit Magic, Divine Magic, Wizardry (formerly called Sorcery) and Ritual Magic. Anyone familiar with any version of RuneQuest will be recognize with the first three. Spirit Magic, as the name implies, deals with the Spirit World. Its practitioners are typically called shamans and they communicate with and bind into service various types of spirits including demons, elementals, ghosts, beast spirits and even spirits contained within inanimate objects such as weapons and armour. Not surprisingly, Divine Magic is derived from the power of the gods and includes a lot of spells involving healing, illusions and natural forces. Wizardry derives its power from scholarly study of the universe. It demands greater commitment from its practitioners than other forms of magic, but can become the most powerful. There are a wide variety of spells in Wizardry including many that resemble those of the other types of magic and is the tradition of choice for anyone dedicated to serious sorcerous power.
The last type of magic is Ritual Magic. Rather than being derived from a different source of power, it represents a different way of spell casting that can be practiced by any sort magic-using character. Powerful summoning, enchantment and divination spells typically involve time-consuming magical procedures, called rituals, to cast.
The Magic Book is an excellent supplement to the BRP core rules. Although it could be used along with the magic system already included in the game, The Magic Book provides a self-contained set of rules that are far superior. I suspect most gamemasters will prefer to simply use them in place of those provided in the BGB.