U could also give each hero six items that have unlimited charges with no cooldown and can't be dropped. The items themselfs would be dummies. Then u can catch the event of when one of these items is used and then trigger actions.
Because u can use the items with the keyboard, that would give u 6 extra keys u can use.
About the spellbook:
the spellbook is an ability. u can add other abilities to it and then add the spellbook to the abilities of your hero.