I like the idea of casting from the spell book. It's organized already. If there were some right click options, it would do the trick. You'd probably want some button or command that would pop up the spell book for only the character whose turn it is.
Something like that might work with inventory items as well. Maybe it pops up separated by item type. The right click options are already there. Throwable items have Throw. Things like arrows have shoot as well as throw, and consumables have eat as well as throw.
The only downside of using the existing panels is that they're large, taking up a quarter of the screen, presumable on the right or left (with an option). I suppose it wouldn't matter if they were for selection only. For example, select Spells or Inventory (button or command), and the panel for the character whose turn it is pops up. If you left click, it's the same as selecting an entry from the HotBar. You;d get the default action. If you right click, you'd have more options. Once you select the item, the panel disappears and you have your full screen back. And if you change your mind, you ccan always right click to cancel the action.
