Modifying the Spell Book
You can change the position of spells, and even delete them permanently from your book.
In order to move a spell, or swap the position of two spells, right-click either slot. Its border in the book will now flash yellow. Right-click the spot in your book where you want it to be, and they will trade places. The same technique can be used to move a spell to an empty slot. You can also start by right-clicking an empty slot if that's more convenient.
In order to delete a spell from your book permanently, first right-click it. Its border in the book will now flash yellow. Next, hit the Delete key, and you will get a confirmation prompt. Click Yes, and the spell will be permanently deleted.