I think that Skyrim has really found perfect balance with Spell Books and Scrolls. All the spells can be learned from Spell Books, we can even disable default spell buying menu and replace it with a selection of spell books, so we don't need to mess around with internal settings too much.
Spell Book - learn the spell. Scroll - cast the spell once. They might have some requirements to use, like magic skill or intelligence, that would certainly be interesting.
I am in favor of adding a whole bunch of new interesting spells, but I don't see how learning multiple spells at a time is beneficial. Not only it can be confusing, so you forget half the spells you learned, it also seems quite OP and not so rewarding. Let each spell be interesting enough to be discovered on their own, not in wholesale bunches.
As for player's spellbook in inventory, I really do not see any point in keeping it. Yes, it was in the vanilla game, but did it really play any role at all? Don't see any use for it, unless you get rid of it to play with no magic, but you can just not use any magic.
Talk about the mods or features you'd like to see in Daggerfall Unity. Give mod creators some ideas!