I try to have all my cels in bags in the cel 
books. It *is* possible (if you don't have the 
cels in a bag) for the cel to adhere itself to 
the black paper in the book's sleeves (the same 
way it would stick to a sketch). Also, cels will 
often slide around in a cel book and it is 
possible for a cel to build up small abrasions on 
it's surface from lots of sliding around (even 
though the plastic pages feel smooth to your 
hand).

But if you buy the acid-free books, there should 
be no trouble with keeping the cel in the book 
without a bag. I would then only suggest that you 
cut up cel bags to put between the cel and the 
black paper inserts in the book's sleeves. Also, 
that you buy a cel book that fits the cels as 
exactly as possible to prevent sliding around.

Of course, this is all coming from someone who 
still keeps some of her cels in one of Yann's 
boxes. ^_^  So take it as you will. ^_~

Many Sharp Smiles,
--Drac