A spellbook is a wizard's record of his lessons learned and knowledge gathered as a magician. The most recognizable function of a spellbook is to record the formulae for the various spells a wizard knows. In addition the wizard will generally add notes about the functions of various spells, observations about how different spells behave. As this information accrues the most senior of wizard's spellbooks can extend into multiple volumes. However, even the newest apprentice will generally have at least a modest book filled with the small spells his master has shared and their observations regarding magic. While a spellbook is less intrinsic to an individual's magic than their athame, it is no less necessary to reach the heights of wizardly power as it can be used to refresh a wizard's memory regarding the less frequently used powers at their disposal, thus providing them greater versatility and the ability to keep more high level spells than they might otherwise be able to remember. The notation in a wizard's spellbook can be unique and intensely personal, some masters requiring that their apprentices learn new spells through description and observation and then record the spells on their own in their own words. Other masters are less stringent in this requirement, which may create patterns of notation that become consistent from master to student.
Wizards tend to be extremely protective of their secrets; many protect their spellbooks with various protective spells, and even those who don't frequently employ codes and symbols designed to hide their secrets (the use of the athame, for example) from prying eyes.