That little black book
A Grimoire is more than a spellcaster’s spell book. It is a running diary in which the caster records all of his occult lore. The Occultist begins play with a Grimoire (generally a gift from his master or a more experienced mentor). Other spellcasters need to construct their own grimoire’s from scratch. The Grimoire records all of the caster’s known rituals, potion recipes, talisman seals, and any various bits of legend and lore she comes across.
In order to prepare spells, a spellcaster studies and meditates upon the information in her Grimoire. Spellcasters do not “forget” their rituals once cast, and do not need to memorize selected rituals before they can be used. Instead, it is better to think of this study as a refresher each day. Like a skilled surgeon who continues to read trade journals in order to keep himself fresh, spellcasters study their Grimoires daily to keep themselves sharp. If a spellcaster loses her Grimoire or is unable to study it, she can still cast her known rituals but suffers a -8 penalty on her Occult checks to cast them.