You have mastered several spells and inscribed them in your spellbook. You start out with three first level spells in your spellbook as well as the cantrips. Whenever you gain a level, you add a new spell of your level or lower to your spellbook. You spellbook is 1 weight.
The information on this site is based on the works of Sage LaTorra and Adam Koebel.
More information can be found at the Dungeon World Official Website.