(Note: This link will expire on 9/2/17)
A spellbook. A book of shadows. A grimoire. These are all names for the same thing: a formula for casting a spell.GURPS Thaumatology: Ritual Path Magic has rules for grimoires to use with the eponymous system, but that has a few assumptions built in from the book it originally appeared in (GURPS Monster Hunters 1: Champions ).