I have the attunement sphere and the Runed Lexicon that I looted off Septimus, and they cannot be removed from the inventory. I know they aren't supposed to weigh anything but I think the Runed Lexicon could look awesome in my house someday! What is the command to make quest items nonessential, if there is one?

Not yet.. as there is no way to detect if the "Quest Object" flag has been checked for the object in the quest alias.