Opening an entire spellbook takes way too much place on the screen. Going through intrusive menus during combats would be boring.
Definitely only show the memorized spells when casting. Everything else is useless information. Preparation UI is the only place where you need to see everything.
I'd like something like this:
Click [Spells] > memorized spells appear as icons in a vertical line above the main UI, grouped by level, and on top of each spell icon we can have smaller icons for level 2, level 3 etc. for upcasting.
You could leave the cast menu open if you want to cast spells with one click.
