Yes that would be a newbie question.
I'm a bit amazed you managed 100 hours without knowing about spellbooks.
After all, while many classes get their spells at levelup and they are always prepared (Sorcerer, Warlock, Bard), you have to prepare spells the very same way on Druid and Wizard, too.
Did you never for example wonder why you could never pick those spells from a Wizard you just choose at levelup ? Or that you learned a spell from a scroll on a Wizard and yet couldnt ever cast it ?