You probably already figured it out by now...
It's really easy.
You're obviously in the abilities editor. When you make your "spellbook" ability, make the spell originally Spell Book. Spell Book ability can be found under the Item category. Once you've done that, scroll down to the Data area of the editor and change the Data - Spell List field to the spells you want to have in your spell book. Make sure if you want more than 3 spells in your spell book to change the Maximum and Minimum Spells values.
Hope that helps you :?