Would you consider adding the ability to have a spellbook curio slot. Current hotkeys that advance to previous or next spell would then target 1) current spellbook on the hotbar if you have currently selected one there, or 2) current spellbook in the curio slot if you have not got one selected in a hotbar slot.

Then add a hotkey to fire current curio spell if there is a spellbook present.

Or even the ability to interact with/use the first spell book found in inventory even if it isn't on the hot bar.

Use your offhand