You are right, it's Enter. I wasn't aware of that, because of these reasons:
- All the dialogs before I progress/close with Z
- On your game page it says Enter = pause
- For me the spell book isn't something I would connect with something called "Pause"
One more side note (a typical bug only we guys from central europe stumble upon): There are keyboards where Z and Y keys switch their places. They are called QUERTZ keyboards. The support for them is quite easy: Just trigger the same action connected to the Z key also for the Y key ;-)