In the latest patch notes it is mentioned that spells can be cast without equipping them:

1.2.389

Major Changes
- Spells no longer ever need to be equipped to be cast

How does this work since I have not found how to cast a spell unless it is 'equipped' on the shortcut bar. Or does this change mean something else?

Same, I didn't see the difference.
Maybe it means we can open the spellbook to launch the spells, I always forgot to try this.

I already tried casting from the spellbook, but the only option I see there is ' Drag to Equip'