In BG1 and 2 they had Priest Spells and WIzard Spells. I think Larian is trying to simplify things by putting Cantraps in the Spell Book as well.
I just found out, and maybe I'm the last to know, that if you put a backpack in your HotBar and put things in the backpack, clicking on the backpack opens dialog with all the things in the backpack. You can then right click and you get the context menu with all your options. So that's another way to organize the HotBar. Presumably pouches work the same way. And you could put scrolls in there as well. Now, if there was only some way of gathering spells plus having a context menu...
