Books and slot counts

The bar is three slots if you are not wearing a Curios spellbook. Put a book on and the bar grows to that book’s capacity. A nine-slot book means a nine-slot bar.

Spells already packed in the book lock into the front of the bar. You can cast them. You cannot drag them off in the equip screen. The leftover slots are yours for scrolls you picked up.

Take the book off, or swap to a smaller one, and extra scrolls come back to your inventory. If your inventory is full they drop at your feet. That is annoying once. After that you leave a chest next to the enchanting setup.

Example: you walk around with no book and three player scrolls. You find a spellbook that holds eight. Equip it. The first few slots fill with whatever the book had inscribed. Your three scrolls slide into the free slots if there is room. If the book already filled everything, those scrolls bounce to inventory.

With Spell Codex the same rule holds, except you are placing Codex spells instead of world scrolls. Empty books still expand the bar. Wide books make the Codex action row scroll sideways.