Originally Posted by Craft (Cheese)
I would call it bad form to take away anything the PCs are utterly dependent upon to function as characters, even temporarily. A Wizard's spellbook is a great example of something that should be immune to this, unless you intend to return it before the next time they prepare spells.
The thing is that in both 4E and 5E, wizards aren't utterly dependent on their spellbook (they'll automatically prepare the same spells as yesterday if they don't have their book).
Of course, if your system relies on the Christmas Tree Effect then removing anything becomes problematic, if only for the amount of bookkeeping involved.